docs: update PEP 561 link to HTTPS

[droppingbeans]

Update PEP 561 reference to use canonical HTTPS URL (https://peps.python.org/pep-0561/).

Security best practice - all python.org links should use HTTPS.

[AlexWaygood]

these days we should probably link to the living documentation (https://typing.python.org/en/latest/spec/distributing.html) rather than the PEP (a historical document) -- want to do that while we're here?

[droppingbeans]

Good point — updated to link to the living documentation instead. Thanks for the suggestion!