✨ NEW: Add warning types myst.subtype

[chrisjsewell]

All parsing warnings are assigned a type/subtype,
and also the messages are appended with them.
These warning types can be suppressed
with the sphinx suppress_warnings config option.

[codecov]

Codecov Report

Merging #313 (60f73e1) into master (c868b3a) will decrease coverage by 0.07%.
The diff coverage is 68.75%.

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
- Coverage   90.65%   90.58%   -0.08%     
==========================================
  Files          14       14              
  Lines        1809     1826      +17     
==========================================
+ Hits         1640     1654      +14     
- Misses        169      172       +3     

Flag	Coverage Δ
pytests	90.58% <68.75%> (-0.08%)	⬇️

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Impacted Files	Coverage Δ
myst_parser/html_to_nodes.py	89.79% <0.00%> (-1.88%)	⬇️
myst_parser/myst_refs.py	91.26% <33.33%> (-0.67%)	⬇️
myst_parser/docutils_renderer.py	91.93% <75.00%> (+0.07%)	⬆️
myst_parser/sphinx_renderer.py	97.43% <81.81%> (-0.54%)	⬇️

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[AakashGfude]

@chrisjsewell this merge is throwing the error, https://github.com/executablebooks/jupyter-book/pull/1167/checks?check_run_id=1900524561 for pdf build.

domain.roles have the following value during the run:

{'': <function paste_any_role at 0x1124b74c0>, 'any': <function paste_any_role at 0x1124b74c0>, 'text': <function paste_text_role at 0x1124c1790>}