Drop Python 2.7 support

[nitzmahone]

PyYAML 5.4 is the last planned release with Python 2.7 support. With 5.4's conversion to setuptools, the python_requires metadata is now being properly generated into the sdists and indexed by PyPI, so Python 2.7 users will be properly directed to a 5.4 release by, eg, pip install pyyaml without needing to pin a version.

Dropping 2.x support will allow us to remove the bifurcated 2.x/3.x codebase- a significant maintenance hassle, as well as simplifying the Windows wheel builds (making them easier to migrate to GHA for a single CI platform).

If major bugs/security/whatever issues are found, we've not ruled out the possibility of doing additional patch releases on the 5.4 codebase, but those would be evaluated case-by-case if/when they happen.

[ingydotnet]

@Thom1729 You should start by removing lib/ and tests/lib and any file references to them.

Then make sure tests run.

Then rename **lib3 to lib and all refs etc, and get tests passing.

Please target any PRs against the release/6.0 branch.

[nitzmahone]

implemented by #550