use intersphinx crossrefs to stdlib docs where possible instead of hardcoded URLs

[hoefling]

This PR replaces hardcoded links to docs.python.org by intersphinx crossrefs where possible. The only URL that was left hardcoded is in how-to/unittest.rst, since the referenced section in stdlib docs (link) doesn't have any label.

[Zac-HD]

Looks good to me 👍

[Zac-HD]

Thanks for catching this! Should we consider adding a CI check (git grep ...?) to make sure we don't introduce new cases?

We might also consider using extlinks so that we can write :issue:'1234' instead of the long link syntax (Hypothesis example), but IMO that's a separate PR.

[hoefling]

@Zac-HD

Should we consider adding a CI check (git grep ...?) to make sure we don't introduce new cases?

Good idea! I'm using an extra sphinx builder (similar to linkcheck builder) for that in my projects, which checks all hardcoded links against hosts in intersphinx_mapping. Can add another PR that introduces this. IIRC there are also cases where hardcoded links can be replaced by intersphinx mappings that should be added first (e.g. refs to attrs or setuptools), but I didn't want to bloat the PR up any more.

We might also consider using extlinks

Yep, totally 👍

[RonnyPfannschmidt]

lovely ideas, they will make fine follow-ups
do you want to create issues for them?

[Zac-HD]

Issue opened! I particularly want to hear more about the builder; that sounds like a trick I'll want to copy for Hypothesis.