feat: Add docutils_compat, fix warning

[tony]

After upgrading libvcs's docutils to v0.19, pytest has a warning:

.venv/lib/python3.10/site-packages/doctest_docutils.py:328: 19 warnings
  ~/libtmux/.venv/lib/python3.10/site-packages/doctest_docutils.py:328: PendingDeprecationWarning: nodes.Node.traverse() is obsoleted by Node.findall().
    for idx, node in enumerate(doc.traverse(condition)):

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

[codecov-commenter]

Codecov Report

Merging #15 (215fd78) into master (74f6f15) will increase coverage by 0.12%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   60.48%   60.60%   +0.12%     
==========================================
  Files           9       10       +1     
  Lines         625      632       +7     
==========================================
+ Hits          378      383       +5     
- Misses        247      249       +2     

Impacted Files	Coverage Δ
src/docutils_compat.py	66.66% <66.66%> (ø)
src/linkify_issues.py	95.74% <100.00%> (+0.09%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more