[Python] Regexp backref identifiers

[michaelblyons]

Centralizes named capture group identifiers in a variable. Adds support for capital letters one place they were missing. Adds support for \g<...> backref style.

Relevant documentation in v3.12

Previously included (but now removed) deprecation scopes for non-ASCII capture group identifiers.

[deathaxe]

Python always used to try to even correctly highlight py2. Do we need to bother users with deprecated highlighting, really?

[michaelblyons]

I can yank the deprecated highlighting and tests, but there's a real change that is still necessary.

I discovered it trying to name the groups in a capture as specific keys for groupdict().

[deathaxe]

I am fine with required fixes, if current patterns don't match all valid names.

It's just the attempt to do linting by scoping no longer supported nonascii "deprecated". pyright and co. will already highlight them depending on set python version. They should however be highlighted normally by syntax as they are valid in older (even ST's default python versions) - even though actually rather unlikely to appear in real world projects, which is just another reason for not needing that extra complexity.

[deathaxe]

Following example from LaTeXTools would be fixed by this PR.