Fix draw_svg warning for mutations above unplotted nodes

[Duncan-JR]

Description

Fixes #2870.
I changed the warning module and modified all the tests that raised that warning to catch it correctly. I made the stack level 2 simply because that seemed to be common with UserWarning instances elsewhere in the package; I'm not sure how one determines that here.

PR Checklist:

• Tests that fully cover new/changed functionality.
• Documentation including tutorial content if appropriate.
• Changelogs, if there are API changes.

[codecov]

Codecov Report

Merging #2871 (90c4207) into main (b06a718) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2871      +/-   ##
==========================================
- Coverage   89.69%   89.69%   -0.01%     
==========================================
  Files          30       30              
  Lines       30160    30159       -1     
  Branches     5860     5860              
==========================================
- Hits        27053    27052       -1     
  Misses       1778     1778              
  Partials     1329     1329              

Flag	Coverage Δ
c-tests	86.09% <ø> (ø)
lwt-tests	80.78% <ø> (ø)
python-c-tests	67.89% <0.00%> (-0.01%)	⬇️
python-tests	98.92% <100.00%> (-0.01%)	⬇️

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Files	Coverage Δ
python/tskit/drawing.py	99.23% <100.00%> (-0.01%)	⬇️

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[benjeffery]

Generally in the test we're using pytest.warns as it's a bit cleaner and lets you check the message: https://happytest.readthedocs.io/en/latest/warnings/#warns

[jeromekelleher]

Yep lgtm

[Duncan-JR]

I've switched the tests to use pytest.warns: it should be ready to merge now

[benjeffery]

Thanks @duncanMR!