Initial fixes for Python 3.14

[cdce8p]

Start fixing some Python 3.14 errors. Tested against 3.14.0b1 locally. Mainly

• Adjust checkers to handle the "new" Union inference result. I.e. it's now a ClassDef instead of a FunctionDef and the builtins type is Union instead of UnionType.
• Remove typing.ByteString and collections.abc.ByteString as these have been removed in 3.14. It's probably not worth it to add separate test cases just for them.
• Also remove typing.re.Pattern and typing.re.Match which have been removed back in 3.13. pylint doesn't emit no-member warnings for these since the typing module has a __getattr__ function.

[github-actions]

🤖 According to the primer, this change has no effect on the checked open source code. 🤖🎉

This comment was generated for commit f86b527