PR summary

• Why is this change necessary?

Unit test coverage for GTK (both v3 and v3) in our GitHub Actions CI tests.yml workflow is largely skipped at the moment.

This is because recent versions of PyGObject -- as required for import gi to succeed -- are incompatible with the girepository-1.0 system dependency that we install on our Ubuntu 22.04 runners.

Specifically, as noted in #29844, PyGObject v3.52 adds a requirement on girepository-2.0.

The goal here is to re-enable GTK-related test coverage.

• What problem does it solve?

To work around the problem, this pull request suggests pinning the upper-bound version of the PyGObject library that we install on the affected Ubuntu 22.04 runners.

• What is the reasoning for this implementation?

1. The girepository-2.0 system dependency is unavailable on Ubuntu 22.04 (jammy) and may not become available on that OS version.

2. We have existing test coverage of PyGObject v3.52 on Ubuntu 24.04 with girepository-2.0 - so my reasoning (also described here) is that retaining coverage of girepository-1.0 is worthwhile.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is (will be, in this case) tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines

Resolves #29844.

Edit: attempt to clarify the nature of the problem that is being solved by this.
Edit: plurality fixup
Edit: rephrase description again.
Edit: clarify that this may not be a temporary fix (girepository-2.0 may not become available on Ubuntu 22.04).