Fix check() and bump to 0.23.6

See python-pytest#1 (comment 171949)

1. First commit fixes the ModuleNotFoundError: No module named 'hypothesis.internal' error when running the check() hook.
   Instead of adding the site-packages dir to $PYTHONPATH, simply add the $PWD to $PYTHONPATH and run pytest with the test dir as first argument.
2. Second commit bumps to the latest version of pytest-asyncio.
   • https://github.com/pytest-dev/pytest-asyncio/releases/tag/v0.23.5
   • https://github.com/pytest-dev/pytest-asyncio/releases/tag/v0.23.6