Update Sphinx to 5.1.1

[MapleCCC]

This pull request should fix #747.

I have tried to build document of LibCST with Sphinx 5.1.1 locally. It works fine and smoothly. The only trivial change we need to make to accommodate is to specify the language setting to non-None value in conf.py.

[codecov-commenter]

Codecov Report

Merging #748 (418fc14) into main (977504f) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #748   +/-   ##
=======================================
  Coverage   94.82%   94.82%           
=======================================
  Files         247      247           
  Lines       25799    25799           
=======================================
  Hits        24463    24463           
  Misses       1336     1336           

Impacted Files	Coverage Δ
libcst/metadata/full_repo_manager.py	97.05% <ø> (ø)

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[MapleCCC]

The CI encountered errors when installing the lxml library in the environment of CPython3.11+Windows. This is due to that lxml library has not published built wheels for the environment of CPython3.11+Windows. Building lxml locally from scratch has always been considered a hard task.

I believe we are safe to ignore these lxml-related CI errors. The PR is now ready to merge, whenever you feel convenient.