fix: Remove cython from install_requirements

[C0rn3j]

This project does not require cython to run from my understanding.

But since it is included as a runtime dependency, pip check fails on it:

% pip check
buildozer 1.5.1.dev0 requires cython, which is not installed.

This PR thus removes it from the deps.

To address the removed comment - if a downstream project still requires cython to be installed as a runtime dependency, add a runtime dependency to the downstream project instead of implementing workarounds elsewhere.

If kivy-ios still requires cython, it's missing in https://github.com/kivy/kivy-ios/blob/892c4b72ee4eb3537310b0694d5fb73cb6a18d53/requirements.txt

If python-4-android still requires cython, it's missing in https://github.com/kivy/python-for-android/blob/d156b3df891b54239d039c3f5af0dc8d42117b0e/setup.py#L21-L25

The Dockerfile even tries to hack its own Cython version in(why, it's currently pulling it from the dependencies, no?), I presume that is equally wrong but wasn't sure, so this PR doesn't address it - I can tack it on if needed:

buildozer/Dockerfile

Line 84 in 105543d

RUN pip install --upgrade "Cython<3.0" wheel pip ${SRC_DIR}