Limit test suite Workflow to only run on changes to Python files

[shenanigansd]

Add a filter to testsuite.yml to only run when .py files are changed.

This should cleanly coexist with the existing branch filter:

If you define both branches/branches-ignore and paths/paths-ignore, the workflow will only run when both filters are satisfied.
- https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#onpushpull_requestpull_request_targetpathspaths-ignore

ref: https://discord.com/channels/267624335836053506/1253355750684753950/1298279250830557236

[shenanigansd]

Hmm, tests are a required check. Since we skipped them, that check will never pass. This is awkward.

[nedbat]

The tests are skipped if the branch is named *notests*, can we use the same mechanism?