Infra + many PEPs: Add Sphinx Lint and fix errors

[hugovk]

Lint PEPs using Sphinx Lint via pre-commit, already used for the devguide.

Let's run it with two checks disabled:

• trailing-whitespace: there's 300+ of these, probably not worth fixing
• role-with-double-backticks: temporarily disabled due to a false positive, reported at False positive: role-with-double-backticks sphinx-contrib/sphinx-lint#34

This PR also fixes the errors it finds.

Currently Sphinx Lint only runs on .rst files; I also fixed the errors it would find if the old .txt PEPs were renamed to .rst (which may happen as part of #10).

Whilst we're editing these PEPs:

• Remove old Version and Last-Modified headers
• Remove old emacs metadata
• Redirect old BPO links to GH issues

[AA-Turner]

Looks OK

A