bpo-36645: Fix ambiguous formatting in re.sub() documentation.

[mollison]

This PR also needs to be backported to 3.7.

https://bugs.python.org/issue36645

[serhiy-storchaka]

Are there precedences of using versionchanged with the same version multiple times sequentially?

[mollison]

@serhiy-storchaka yes. Here are some examples.

versionchanged::3.7 repeated sequentially:
https://docs.python.org/3/library/collections.html#collections.namedtuple

versionchanged::3.2 repeated sequentially:
https://docs.python.org/3/library/urllib.request.html#urllib.request.urlopen

versionchanged::3.7 repeated sequentially:
https://docs.python.org/3/library/dis.html#dis.dis

If you want to look at this issue in greater depth, subprocess.Popen is a more complicated case:
https://docs.python.org/3/library/subprocess.html#subprocess.Popen

[berkerpeksag]

I agree that multi-line version{added,changed} markers don't render well:

This PR makes the note more readable.

LGTM, thank you!

[miss-islington]

Thanks @mollison for the PR, and @berkerpeksag for merging it 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[bedevere-bot]

GH-12898 is a backport of this pull request to the 3.7 branch.