Support Cython 3

[Julian-O]

[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.

[misl6]

The major issue is not Kivy or packages maintained by us, but packages from outside of the Kivy ecosystem.

What we did not in past, and we should do now (as tech was not here, and the whole Python ecosystem did not), is to isolate the build environment for each python package recipe.

That will make possible to use a different version of build-time dependencies, for each recipe. (And that's what happens now while building a wheel via pip install packagename for desktop platforms)

[GvozdevLeonid]

Are there any plans to add cython3+ support?