Building on PyPy

[apmorton]

conda-forge/aiocsv-feedstock#1

Py_UNREACHABLE is apparently not defined by pypy, and this causes the build to fail for pypy.

[MKuranowski]

Feel free to send patches for PyPy compatibility, however, do measure if the python parser implementation is also unbearably slow and the C extension is necessary.

Py_UNREACHABLE has been added in CPython 3.7, so if PyPy strives for full C api compatibility, it's an omission on their part.

I have considered in the past changing the code to only use the limited C API, but iirc necessary features were only added in 3.10, which conflicts with my desire to support all supported minor Python releases.