⬆️ Drop Sphinx 3, add Sphinx 5

[chrisjsewell]

No description provided.

[codecov]

Codecov Report

Merging #579 (9a6ed16) into master (3c45b7e) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #579      +/-   ##
==========================================
+ Coverage   89.83%   89.86%   +0.02%     
==========================================
  Files          20       21       +1     
  Lines        2116     2122       +6     
==========================================
+ Hits         1901     1907       +6     
  Misses        215      215              

Flag	Coverage Δ
pytests	89.86% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
myst_parser/_compat.py	100.00% <100.00%> (ø)
myst_parser/mdit_to_docutils/base.py	92.27% <100.00%> (+0.01%)	⬆️
myst_parser/sphinx_ext/myst_refs.py	90.44% <100.00%> (+0.07%)	⬆️

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