> If you take this route, please don't write it directly against the CPython C-API (as you would for a CPython stdlib module).

Thanks for reminding me of this. I was planning to take the laziest route possible anyway, reusing as much code from cpython as possible.

I will probably start with a minimal implementation of these base classes https://github.com/python/cpython/blob/master/Lib/_compression.py using cython, using the gzip implementation in cpython as guidance.