Replace deprecated logging.warn with logging.warning

[hugovk]

Like pytest-dev/pytest#10064.

Replace logging.warn (deprecated in Python 2.7, 2011) with logging.warning (added in Python 2.3, 2003).

• https://docs.python.org/3/library/logging.html#logging.Logger.warning
• logging.warn() is not documented python/cpython#57444

[hugovk]

Also fix Error: PyPy 3.6 not found on CI, and bump actions versions.

Re: https://github.com/actions/setup-python#specifying-a-pypy-version

Edit: hang on, need to fix that properly! Done!

[RonnyPfannschmidt]

thanks!