🔧 Update mypy to use sphinx v7.3

[chrisjsewell]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 90.14%. Comparing base (167c902) to head (9ce2d92).

Files	Patch %	Lines
myst_parser/sphinx_ext/myst_refs.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #917   +/-   ##
=======================================
  Coverage   90.14%   90.14%           
=======================================
  Files          24       24           
  Lines        3400     3400           
=======================================
  Hits         3065     3065           
  Misses        335      335           

Flag	Coverage Δ
pytests	90.14% <66.66%> (ø)

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