stdlib/xml/sax: Add type annotations

[Screwtapello]

This doesn't add complete type annotations, but it improves things for the most commonly used objects in the SAX API.

[Screwtapello]

It looks like a bunch of CI tests failed because third-party-library stubs that subclass xml.sax.handler.ContentHandler and override startElement() don't actually use the correct annotations for the attrs parameter.

• the docs describe an interface called "Attributes", which is superset of the Mapping[str, str] interface, but no such class exists in the code (probably because the code pre-dates ABCs)
• the implementation always uses an instance of AttributesImpl, which implements the "Attributes" interface, but of course doesn't actually declare it anywhere (or mention Mapping).
• My type annotation says the attrs parameter will always be of type AttributesImpl (which is true) but of course that conflicts with everyone that just read the documentation and said "sure, Mapping[str, str] is close enough"

I'm not sure what clever type machinery exists to solve this (type aliases? ABCs? Protocols?) but I'm happy to try things if someone can provide guidance.

[github-actions]

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

[JelleZijlstra]

Thanks, I think your solution makes sense.