Improve PyPI READMEs for stubs packages

[AlexWaygood]

• Link directly to PEP-561
• Link directly to the runtime package the stubs are for, if typeshed provides the upstream_repository field in a stubs package's METADATA.toml file.
• Clearly state that types-requests==2.31.0.8 aims to provide accurate annotations for requests>=2.31.0.

[AlexWaygood]

We might want to switch to using jinja for some of the stuff going on in build_wheel.py; it can make stuff like this much more readable, in my opinion.

[AlexWaygood]

Possibly we could do some validation here. Typeshed already does a bunch of validation checks for this field, though, so maybe it's not necessary.

[srittau]

A few general notes: Generally the stub_uploader should treat everything coming from typeshed as "tainted" as a precaution against an attacker gaining access to a typeshed maintainer's GitHub account. (This is the reason why we have so few stub_uploader maintainers -> to limit the attack vector. Personally, I consider every typeshed maintainer to be a stub_uploader maintainer, just without the commit rights.) Crashing the uploader is fine, the risk is uploading potentially harmful code under a "types-" name.

That said, an isinstance check and regexp with "safe" markdown chars can't hurt here. The regexp check should probably just print a warning and return None.

[AlexWaygood]

I added a little more validation in 796d2ea. We could add more. Should we return None if .isascii() returns False? If a regex, what regex?

Open to ideas here, not sure how far we should go :)

[srittau]

Nice, these are very useful additions!