Remove workaround for fixed Python zipfile issue.

The issue that was worked around has been fixed in upstream CPython:
python/cpython@36ff513

Since the fix is included in CPython v3.8.4+ and v3.9+, which includes
all supported versions, we can get rid of this ugly workaround.

full_offline_backup_for_todoist/virtual_fs.py

@@ -34,8 +34,6 @@ def write_file(self, file_path: str, file_data: bytes) -> None:
 
 class ZipVirtualFs(VirtualFs):
     """ Represents a virtual filesystem over a ZIP file """
-    __ZIP_FLAG_BITS_UTF8 = 0x800
-
     src_path: Optional[str]
     dst_path: Optional[str]
     _zip_file: Optional[zipfile.ZipFile]
@@ -88,28 +86,3 @@ def read_file(self, file_path: str) -> bytes:
     def write_file(self, file_path: str, file_data: bytes) -> None:
         assert self._zip_file
         self._zip_file.writestr(file_path, file_data)
-
-        # WORKAROUND FOR A PYTHON BUG in Python's zipfile (at least in Python 3.6.5)
-        # There's a bug in the zipfile module, where if a file with a non-ASCII name
-        # is first writen to the ZIP, and then immediately read before saving the ZIP,
-        # then it will crash with something like:
-
-        # zipfile.BadZipFile: File name in directory '🦋' and header b'\xf0\x9f\xa6\x8b' differ.
-        # Sample reproduction:
-        #    with zipfile.ZipFile(io.BytesIO(), "a") as zip:
-        #    zip.writestr("🦋test", b'010203')
-        #    print(zip.read("🦋test"))
-        # This happens because the flag_bits specifying whether the filename is UTF-8 or not
-        # are set when saving the ZIP, not when creating the in-memory ZipInfo instance,
-        # so when reading the file again in-memory, it gets confused and thinks the filename
-        # in the ZipInfo is not UTF-8 and tries to decode it with another encoding
-
-        # This isn't really surprising because encoding in zipfile.py is a mess currently...
-        # See also: https://bugs.python.org/issue12048 https://bugs.python.org/issue10614
-
-        # As a workaround, we set the in-memory ZipInfo flag when the filename is UTF-8
-        try:
-            file_path.encode('ascii')
-        except UnicodeEncodeError:
-            file_info = self._zip_file.getinfo(file_path)
-            file_info.flag_bits = file_info.flag_bits | self.__ZIP_FLAG_BITS_UTF8