ci: force agg backend on windows and fix whl version

[yuzhichang]

• Does this PR have a descriptive title that could go in our release notes?
• Does this PR add any new dependencies?
• Does this PR modify any existing APIs?
  • Is the change to the API backwards compatible?
• Have you built the documentation (reference and/or tutorial) and verified the generated documentation is appropriate?

Reference Issues/PRs

What does this implement/fix? Briefly explain your changes.

Fix whl version error:

• https://github.com/graspologic-org/graspologic/actions/runs/19643759002/job/56254810921

Ensured both test-coverage and unit-and-doc-test jobs export MPLBACKEND=Agg before invoking uv run …, so Matplotlib stays headless and the Windows runners no longer touch the broken Tk install. This fix graspologic Reporting errors:

• https://github.com/graspologic-org/graspologic/actions/runs/19577011727/job/56064948269
• https://github.com/graspologic-org/graspologic/actions/runs/19641687256/job/56246548280
• https://github.com/graspologic-org/graspologic/actions/runs/19641687256/job/56246548265

Any other comments?

[bdpedigo]

thanks @yuzhichang