Remove unused size.py and refs to removed CPython files

[hugovk]

size.py was added in 2010 (079bd65), last had functional changes in 2013 (cf79988) and isn't used by any other release scripts.

Include/Python-ast.h last existed in 3.9, and doesn't exist in 3.10+:

• https://github.com/python/cpython/blob/3.9/Include/Python-ast.h
• https://github.com/python/cpython/tree/3.10/Include
• python/cpython#24933

Other dotfile removals:

• 2017: .hgtouch python/cpython#1405
• 2017: .codecov.yml was renamed to .github/codecov.yml (python/cpython#1494) and removed in 2021 (python/cpython#25679)
• 2017: .bzrignore python/cpython#1661
• 2017: .mention-bot python/cpython#2923
• 2018: .hgeol, .hgignore and .hgtags python/cpython#5558
• 2021: .travis.yml python/cpython#30257

Docs tools:

• We used to have some vendored docs tools:
  • tools/docutils
  • tools/jinja2
  • tools/pygments
  • tools/sphinx
• They were checked out from SVN but removed in 2014: python/cpython#f7b2f36
• We can probably drop the venv removal, as it was created as a tempfile.TemporaryDirectory()
• In fact, I suspect we can remove the whole not skip_docs branch of export(), as we only call it in CI via python ../release.py --export "$CPYTHON_RELEASE" --skip-docs as part of build-source. build-docs doesn't call this function, it uses the Makefile in Doc instead. This can wait for another time.