Issue #41: strip Sphinx cross-referencing syntax from reason in warning message

[soxofaan]

Addresses issue #41 : strip sphinx cross-referencing syntax from reason in warning message.

Commit checklist:

• [x] add tests that fail without the patch
• [ ] ensure all tests pass with pytest
• [x] add documentation to the relevant docstrings or pages
• [x] add versionadded or versionchanged directives to relevant docstrings
• [x] add a changelog entry if this patch changes code

[soxofaan]

FYI: some tests seem to be failing (also when I run locally), but that also happens without my patch