diff --git a/src/h2/connection.py b/src/h2/connection.py
index 086112eec..ca2b38329 100644
--- a/src/h2/connection.py
+++ b/src/h2/connection.py
@@ -33,7 +33,7 @@
 from .frame_buffer import FrameBuffer
 from .settings import Settings, SettingCodes
 from .stream import H2Stream, StreamClosedBy
-from .utilities import SizeLimitDict, guard_increment_window, utf8_encode_headers
+from .utilities import SizeLimitDict, guard_increment_window
 from .windows import WindowManager
 
 
@@ -975,7 +975,6 @@ def push_stream(self, stream_id, promised_stream_id, request_headers):
         )
         self.streams[promised_stream_id] = new_stream
 
-        request_headers = utf8_encode_headers(request_headers)
         frames = stream.push_stream_in_band(
             promised_stream_id, request_headers, self.encoder
         )
diff --git a/src/h2/events.py b/src/h2/events.py
index 66c3cff4a..2c3409424 100644
--- a/src/h2/events.py
+++ b/src/h2/events.py
@@ -591,8 +591,8 @@ def __init__(self):
     def __repr__(self):
         return (
             "<AlternativeServiceAvailable origin:%s, field_value:%s>" % (
-                self.origin.decode('utf-8', 'ignore'),
-                self.field_value.decode('utf-8', 'ignore'),
+                (self.origin or b'').decode('utf-8', 'ignore'),
+                (self.field_value or b'').decode('utf-8', 'ignore'),
             )
         )
 
diff --git a/src/h2/stream.py b/src/h2/stream.py
index 629bbe548..a6b77289e 100644
--- a/src/h2/stream.py
+++ b/src/h2/stream.py
@@ -854,6 +854,7 @@ def send_headers(self, headers, encoder, end_stream=False):
         input_ = StreamInputs.SEND_HEADERS
 
         headers = utf8_encode_headers(headers)
+
         if ((not self.state_machine.client) and
                 is_informational_response(headers)):
             if end_stream:
@@ -1243,6 +1244,9 @@ def _build_headers_frames(self,
         """
         Helper method to build headers or push promise frames.
         """
+
+        headers = utf8_encode_headers(headers)
+
         # We need to lowercase the header names, and to ensure that secure
         # header fields are kept out of compression contexts.
         if self.config.normalize_outbound_headers:
diff --git a/src/h2/utilities.py b/src/h2/utilities.py
index 54cd6f210..1d60f1bd2 100644
--- a/src/h2/utilities.py
+++ b/src/h2/utilities.py
@@ -105,27 +105,26 @@ def extract_method_header(headers):
 
 def is_informational_response(headers):
     """
-    Searches a header block for a :status header to confirm that a given
+    Searches headers list for a :status header to confirm that a given
     collection of headers are an informational response. Assumes the header
-    block is well formed: that is, that the HTTP/2 special headers are first
-    in the block, and so that it can stop looking when it finds the first
-    header field whose name does not begin with a colon.
+    are well formed and encoded as bytes: that is, that the HTTP/2 special
+    headers are first in the block, and so that it can stop looking when it
+    finds the first header field whose name does not begin with a colon.
 
-    :param headers: The HTTP/2 header block.
+    :param headers: The HTTP/2 headers.
     :returns: A boolean indicating if this is an informational response.
     """
     for n, v in headers:
-        # If we find a non-special header, we're done here: stop looping.
+        if not isinstance(n, bytes) or not isinstance(v, bytes):
+            raise ProtocolError(f"header not bytes: {n=:r}, {v=:r}")  # pragma: no cover
 
-        if n and n[0] != SIGIL:
+        if not n.startswith(b':'):
             return False
-
-        # This isn't the status header, bail.
         if n != b':status':
+            # If we find a non-special header, we're done here: stop looping.
             continue
-
         # If the first digit is a 1, we've got informational headers.
-        return v[0] == INFORMATIONAL_START
+        return v.startswith(b'1')
 
 
 def guard_increment_window(current, increment):
@@ -515,14 +514,14 @@ def utf8_encode_headers(headers):
     tuples that preserve the original type of the header tuple for tuple and
     any ``HeaderTuple``.
     """
-    return [
-        (
-            header.__class__(_to_bytes(header[0]), _to_bytes(header[1]))
-            if isinstance(header, HeaderTuple)
-            else (_to_bytes(header[0]), _to_bytes(header[1]))
-        )
-        for header in headers
-    ]
+    encoded_headers = []
+    for header in headers:
+        h = (_to_bytes(header[0]), _to_bytes(header[1]))
+        if isinstance(header, HeaderTuple):
+            encoded_headers.append(header.__class__(h[0], h[1]))
+        else:
+            encoded_headers.append(h)
+    return encoded_headers
 
 
 def _lowercase_header_names(headers, hdr_validation_flags):
diff --git a/test/test_h2_upgrade.py b/test/test_h2_upgrade.py
index 1fa851e90..7954a52cb 100644
--- a/test/test_h2_upgrade.py
+++ b/test/test_h2_upgrade.py
@@ -32,6 +32,7 @@
     (b':method', b'GET'),
 ]
 
+
 class TestClientUpgrade(object):
     """
     Tests of the client-side of the HTTP/2 upgrade dance.
diff --git a/test/test_head_request.py b/test/test_head_request.py
index a804f90a7..2f46b72f2 100644
--- a/test/test_head_request.py
+++ b/test/test_head_request.py
@@ -21,6 +21,7 @@
     (':method', 'HEAD'),
 ]
 
+
 class TestHeadRequest(object):
     example_response_headers = [
         (b':status', b'200'),
diff --git a/test/test_invalid_headers.py b/test/test_invalid_headers.py
index 996e48aa6..f388cf3fb 100644
--- a/test/test_invalid_headers.py
+++ b/test/test_invalid_headers.py
@@ -297,8 +297,9 @@ def test_headers_event_skipping_validation(self, frame_factory, headers):
         c.send_headers(1, headers)
 
         # Ensure headers are still normalized.
+        headers = h2.utilities.utf8_encode_headers(headers)
         norm_headers = h2.utilities.normalize_outbound_headers(
-            h2.utilities.utf8_encode_headers(headers), None, False
+            headers, None, False
         )
         f = frame_factory.build_headers_frame(norm_headers)
         assert c.data_to_send() == f.serialize()
@@ -323,10 +324,12 @@ def test_push_promise_skipping_validation(self, frame_factory, headers):
         )
         c.receive_data(header_frame.serialize())
 
-        # Create push promise frame with normalized headers.
         frame_factory.refresh_encoder()
+
+        # Create push promise frame with normalized headers.
+        headers = h2.utilities.utf8_encode_headers(headers)
         norm_headers = h2.utilities.normalize_outbound_headers(
-            h2.utilities.utf8_encode_headers(headers), None, False
+            headers, None, False
         )
         pp_frame = frame_factory.build_push_promise_frame(
             stream_id=1, promised_stream_id=2, headers=norm_headers
diff --git a/tox.ini b/tox.ini
index 74e0c2191..1d3d7fee3 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = py39, py310, py311, py312, py13, pypy3, lint, docs, packaging
+envlist = py39, py310, py311, py312, py313, pypy3, lint, docs, packaging
 
 [gh-actions]
 python =
@@ -27,7 +27,7 @@ commands = pytest {posargs}
 
 [testenv:lint]
 deps =
-    flake8>=3.9.1,<4
+    flake8>=7.1.1,<8
 commands = flake8 src/ test/
 
 [testenv:docs]