Add preliminary support for Python 3.11

Now that Python 3.10 is officially released and 3.11 is receiving
alpha versions, indicate that we're testing with 3.11 by updating
the trove classifiers for the Python package.

Because PyYAML is not yet building manylinux wheels for 3.11, re-add
the PYTHONWARNINGS exclusion for its Cython language_version
ambiguity (hopefully targeted for a fix soon in v6). Also pin our
minimum PyYAML requirement to the initial v6 beta version in order
to avoid needing to reintroduce the license_file DeprecationWarning
exclusion.