🔧 MAINTAIN: Minor fixes

[chrisjsewell]

Improve integration with myst-nb

[codecov]

Codecov Report

Merging #585 (b3b7d68) into master (75ef9cb) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master     #585   +/-   ##
=======================================
  Coverage   89.86%   89.86%           
=======================================
  Files          21       21           
  Lines        2122     2122           
=======================================
  Hits         1907     1907           
  Misses        215      215           

Flag	Coverage Δ
pytests	89.86% <50.00%> (ø)

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Impacted Files	Coverage Δ
myst_parser/mdit_to_docutils/base.py	92.27% <0.00%> (ø)
myst_parser/sphinx_ext/myst_refs.py	90.44% <100.00%> (ø)

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