[MNT]: CI: either remove, or separate cairocffi CI paths #29822
Description
Summary
The cairocffi dependency was added to (Travis) CI to support the pgi object bindings (0ed289c) that are no-longer-supported (3221c94).
However: cairocffi will still be used if cairo (e.g. pycairo) is unavailable at runtime:
matplotlib/lib/matplotlib/backends/backend_cairo.py
Lines 15 to 26 in 6fc8169
We currently install cairocffi during GitHub Actions CI unit testing; so there is a possibility that it would mask failures to import and exercise pycairo-based test coverage.
NB: This is my current understanding as I explore #29732 - I'm not 100% confident about my findings.
Proposed fix
Ideally, I think we should extract a separate cairocffi-based GitHub Actions CI unit test matrix job, so that both pycairo and cairocffi are exercised.
As a fallback if that proves infeasible, we could remove cairocffi from the dependencies installed during CI workflows.