Add extract_docstrings script to sdist

[N-Wouda]

This PR:

• Closes Build error during installation on MacOS #374.
• Adds and passes relevant tests.
• Has well-formatted code and documentation.

Notes:

• It is essential that you add a test when making code changes.
  This keeps the code coverage level up, and helps ensure the changes work as intended.
• When fixing a bug, you must add a test that would produce the bug in the master branch, and then show that it is fixed with the new code.
• New code additions must be well formatted. Changes should pass the pre-commit workflow, which you can set up locally using pre-commit.
• Docstring additions must render correctly, including escapes and LaTeX.