Add `kivy==master` (Kivy 3.x) to the test matrix

Kivy development has moved forward significantly and Kivy 3.x is actively developed on `master`. To ensure python-for-android stays compatible with the upcoming major release, it would be valuable to add `kivy==master` as a test matrix entry alongside the current stable pin.

This would let us catch breakages early — on both sides — and give contributors working on Kivy 3.x compatibility a clear signal that p4a is tracking it.

What have to be done:

- [ ] Add a `kivy==master` entry to the CI test matrix
- [ ] Mark it as `allow-failure` / experimental if needed to avoid blocking merges while things are in flux

### Question for the Kivy side

I'm happy to coordinate: **is there anything currently missing in `kivy/kivy` that is currently blocking the integration? I've missed any PR?

Start by locating the existing stable Kivy pin in the CI test matrix and inspect how matrix entries and experimental failures are configured. Add a `kivy==master` entry, allowing failure if appropriate, and verify that the matrix runs the new compatibility job without blocking the stable jobs.

Written by the indexing model from the issue text.