Bump Flake8 to fix CI for Python 3.7

[hugovk]

The Python 3.7 CI job has started failing:

    self._load_entrypoint_plugins()
  File "/home/runner/work/html5lib-python/html5lib-python/.tox/py/lib/python3.7/site-packages/flake8/plugins/manager.py", line 254, in _load_entrypoint_plugins
    eps = importlib_metadata.entry_points().get(self.namespace, ())
AttributeError: 'EntryPoints' object has no attribute 'get'
ERROR: InvocationError for command /home/runner/work/html5lib-python/html5lib-python/.tox/py/bin/flake8 . (exited with code 1)

This is because importlib_metadata 5.0 removes some deprecations, and it's a dependency of Flake8. But new Flake8 works, so let's update:

• 5.0.0 fails with flake8 python/importlib_metadata#406
• flake8 and importlib_metadata 5.0 support PyCQA/flake8#1701

Also fix the new Flake8 findings:

./html5lib/_inputstream.py:327:27: E275 missing whitespace after keyword
./html5lib/serializer.py:225:15: E275 missing whitespace after keyword
./html5lib/serializer.py:232:15: E275 missing whitespace after keyword
./html5lib/treewalkers/etree.py:40:19: E275 missing whitespace after keyword
./html5lib/treebuilders/etree.py:111:19: E275 missing whitespace after keyword
./html5lib/treebuilders/etree.py:204:19: E275 missing whitespace after keyword
./html5lib/tests/test_serializer.py:77:19: E275 missing whitespace after keyword