Declare support for Python 3.11 and use stdlib tomllib

[hugovk]

Declare support for Python 3.11 via Trove classifiers.

Also tomli has been added to the Python stdlib in 3.11, let's use that directly and avoid installing an extra dependency:

• https://docs.python.org/3/whatsnew/3.11.html#summary-release-highlights
• https://docs.python.org/3/library/tomllib.html#module-tomllib
• https://peps.python.org/pep-0680/

[ikamensh]

Thanks for the PR!