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?

首先在 CI 测试矩阵中找到现有的稳定版 Kivy pin，并检查矩阵条目和实验性失败是如何配置的。添加一个 `kivy==master` 条目，在适当的情况下允许失败，并验证矩阵会运行新的兼容性作业，同时不会阻塞稳定作业。

由索引模型根据 Issue 内容生成。