Convert PR numbers in docs/change_log to clickable links

[sumezulike]

Description

Closes #4258.

This adds a listener to the sphinx include-read event to find all PR numbers (#X) with regex and replace them with the corresponding link [(#X)](https://github.com/psf/black/pull/X).

I didn't add an entry to CHANGES.md since this seems like a rather minor docs change.

Checklist - did you ...

• [-] Add an entry in CHANGES.md if necessary?
• [-] Add / update tests if necessary?
• [-] Add new / update outdated documentation?

[JelleZijlstra]

Thanks!

[JelleZijlstra]

I guess we're not formatting this file with Black? Let's fix that in a separate PR though.

[JelleZijlstra]

The preview is at https://black--4346.org.readthedocs.build/en/4346/change_log.html, looks great!

[cobaltt7]

Looks very clean! I skimmed the changelog and I did find a few inconsistencies that weren't always linked properly:

Add extra blank lines in stubs in a few cases (#3564, #3862)
Upgrade to mypy 1.7.1 (#4049) (#4069)
Revert the change ... (#3940) for better compatibility ...  (#4137)
... This fixes issue #1629 and all of its many many duplicates. (#2126)
added `--extend-exclude` argument (PR #2005)
re-implemented support for explicit trailing commas ... (#1288 and duplicates)

It may be easiest to linkify all mentions, even if outside parenthesis.

[JelleZijlstra]

Oops sorry, I merged just as @RedGuy12's comment appeared. Agree it makes sense to linkify all instances of #NNN even if they are not immediately within parentheses.

[sumezulike]

Good point, I overlooked that there were inconsistencies, didn't scroll that far 😄
I initially decided to make it strict because the docs dictate the format like

- `Black` is now more awesome (#X)

Also, some people mention issues in the descriptions which would then cause invalid links.
I can make the matching more lenient though, maybe all #numbers inside parentheses?
So Fixes stuff (#123 and #456) would match both but Fixes #123 (#456) only matches 456?

[JelleZijlstra]

Issue links are actually fine, GitHub redirects them. I put up an implementation in #4347.

[sumezulike]

Alright, thanks for the review!