Improve MyPy configuration

Some improvments flagged by Repo-Review: https://learn.scientific-python.org/development/guides/repo-review/

1. Remove now-default show_error_codes.
2. Enable some optional error codes with extra checks.

pyproject.toml

@@ -87,9 +87,13 @@ source = [
 show_missing = true
 
 [tool.mypy]
+enable_error_code = [
+  "ignore-without-code",
+  "redundant-expr",
+  "truthy-bool",
+]
 mypy_path = "src/"
 namespace_packages = false
-show_error_codes = true
 strict = true
 warn_unreachable = true
 

src/corsheaders/checks.py

@@ -47,7 +47,7 @@ def check_settings(**kwargs: Any) -> list[CheckMessage]:
         )
 
     if (
-        not isinstance(conf.CORS_PREFLIGHT_MAX_AGE, int)
+        not isinstance(conf.CORS_PREFLIGHT_MAX_AGE, int)  # type: ignore [redundant-expr]
         or conf.CORS_PREFLIGHT_MAX_AGE < 0
     ):
         errors.append(