Implement PEP-696

[thereversiblewheel]

Summary

This PR adds the ability for LibCST to parse the "=" operator inside of the square brackets as outlined in PEP-696

Major Changes

Additional python tests were added
Additional roundtrip tests were added (taken from the cython implementation)

##Related Links

PEP-696: https://peps.python.org/pep-0696/#grammar-changes
Issue: #1127

Test Plan

Round trip tests we're added

[codecov]

Codecov Report

Attention: Patch coverage is 91.42857% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 91.15%. Comparing base (9f54920) to head (a4a0147).
Report is 11 commits behind head on main.

Files	Patch %	Lines
libcst/_nodes/statement.py	87.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1141      +/-   ##
==========================================
+ Coverage   91.12%   91.15%   +0.02%     
==========================================
  Files         256      258       +2     
  Lines       26567    26740     +173     
==========================================
+ Hits        24210    24374     +164     
- Misses       2357     2366       +9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[zsol]

lgtm at a high level, I have some minor comments, the most important thing is to fix the ordering of inflate statements.
Also pls add more test cases for roundtrip

[zsol]

Looks great, thanks :)