Support Cython 3

[This is an issue well-known to the developers, but I don't see it documented in the Issues database.]

Cython 3 was released in July 2023.
It isn't compatible with python-for-android and/or its recipes. [I am not familiar with the reasons or results.]
As a result, it has been recommended (and automated in the case of Buildozer' setup.py) to freeze the version of Cython installed to below 3.0.0.

python-for-android should support Cython3.

There was a debate about what the impact of p4a upgrading its needed Cython version would have on users who installed it via Buildozer.

Ideally:
- python-for-android would support both Cython 2 and Cython 3 for a window long enough to include a Buildozer release.
- python-for-android installation (or execution) would take care of Cython dependencies so Buildozer need not.
- if/when python-for-android increases its minimum Cython version, it should give meaningful error messages if the wrong Cython version is provided.

Start with python-for-android's installation or execution dependency handling and the Buildozer setup.py reference in the issue. Review the Cython 2 and Cython 3 compatibility impact, then verify that both versions are supported during the proposed transition and that an incompatible version produces a meaningful error.

Written by the indexing model from the issue text.