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?