Update mkdocs extensions to fix local syntax highlighting

[daviskirk]

Change Summary

Update mkdocs extensions to fix local syntax highlighting.

Ran into #2400 as well.

According to https://squidfunk.github.io/mkdocs-material/reference/code-blocks/
the pymdownx.highlight extension supersedes the CodeHilite extension,
the "extra" extension was still causing the syntax highlighting to disappear (it would show up when taking it out).
But apparently, there's https://facelessuser.github.io/pymdown-extensions/extensions/extra/ which also replaces the "extra" extension, and also fixed the syntax highlighting in combination with pymdownx.highlight.

Related issue number

Fixed #2400

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #2401 (2176043) into master (c8883e3) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2401   +/-   ##
=======================================
  Coverage   99.90%   99.90%           
=======================================
  Files          25       25           
  Lines        5039     5043    +4     
  Branches     1031     1032    +1     
=======================================
+ Hits         5034     5038    +4     
  Misses          1        1           
  Partials        4        4           

Impacted Files	Coverage Δ
pydantic/json.py	100.00% <0.00%> (ø)

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[PrettyWood]

Nice! What about attr_list?

[samuelcolvin]

Thanks so much, it was the extra bit I was missing.

[samuelcolvin]

Nice! What about attr_list?

@daviskirk I think we need to keep attr_list.

update: looks like attr_list is active by default also I don't think we use it

[samuelcolvin]

thanks so much.