Fix the missing source_url warning raised by jaraco.packaging.sphinx

[julianz-]

Version 10.4.0 of jaraco.packaging.sphinx introduced the requirement to provide a source_url. This PR adds it to pyproject.toml.

❓ What kind of change does this PR introduce?

• 🐞 bug fix
• 🐣 feature
• 📋 docs update
• 📋 tests/coverage improvement
• 📋 refactoring
• 💥 other

📋 What is the related issue number (starting with #)

Resolves #

❓ What is the current behavior? (You can also link to an open issue here)
Builds currently fail with a warning:

The config value 'source_url' has type 'NoneType', defaults to 'str'. See e.g. here

The jaraco.packaging.sphinx extension added a requirement to provide a source_url as a change in v10.4.0.

❓ What is the new behavior (if this is a feature change)?

📋 Other information:

📋 Contribution checklist:

(If you're a first-timer, check out
this guide on making great pull requests)

• I wrote descriptive pull request text above
• I think the code is well written
• I wrote good commit messages
• I have squashed related commits together after
  the changes have been approved
• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.15%. Comparing base (4bb134f) to head (dfbaa27).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #817      +/-   ##
==========================================
- Coverage   78.21%   78.15%   -0.07%     
==========================================
  Files          41       41              
  Lines        4788     4788              
  Branches      547      547              
==========================================
- Hits         3745     3742       -3     
- Misses        905      906       +1     
- Partials      138      140       +2