atom build fail

### Checklist

- [x ] the issue is indeed a bug and not a support request
- [x ] issue doesn't already exist: https://github.com/kivy/python-for-android/issues
- [ ] I have a short, runnable example that reproduces the issue
- [x ] I reproduced the problem with the latest development version (`p4a.branch = develop`)
- [x ] I used the grave accent (aka backticks) to format code or logs when appropriated

### Logs

```
[INFO]: Building atom for arm64-v8a
[INFO]: atom apparently isn't already in site-packages
[INFO]: Building compiled components in atom
[INFO]: -> directory context /home/bobf/ex/rtest/.buildozer/android/platform/build-arm64-v8a/build/other_builds/atom/arm64-v8a__ndk_target_21/atom
[DEBUG]: -> running python3 setup.py build_ext -v
[DEBUG]: Traceback (most recent call last):
[DEBUG]: File "/home/bobf/ex/rtest/.buildozer/android/platform/build-arm64-v8a/build/other_builds/atom/arm64-v8a__ndk_target_21/atom/setup.py", line 13, in
[DEBUG]: from cppy import CppyBuildExt
[DEBUG]: ModuleNotFoundError: No module named 'cppy'

```

Reproduce the arm64-v8a atom build from the reported log and inspect atom/setup.py, starting with its import of cppy. Trace how this dependency is handled by the python-for-android build, then confirm the atom build completes without the ModuleNotFoundError.

Written by the indexing model from the issue text.