When the tests are run with `python setup.py test` (e.g. from the ./run-tests.sh
script), we can't have setuptools install pre-compiled wheels for extra test
requirements (e.g. Brotli, unicodedata2).
Instead of failing, it's better that we skip the tests that have such extra
requirements.
NOTE: The recommended way to run the full test suite is by using tox
(like we do on the CI), or by installing pytest and the rests of requirements.txt,
and then run `pytest` command directly.
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