Full PyPy support

pyo3-pack gets basic pypy support from #105, while some things are still missing.

Add some automated testing, ideally on ci as pyo3 does.
Figure out the best way to deal with manylinux.
- pypy doesn't support maynlinux1. manylinux2010 was mentioned to be on the agenda of a pypy sprint meeting, however I couldn't find anything about the results.
- If we need to keep different defaults for pypy and cpython, the Manylinux enum should get a default variant so we can disambiguate whether a user requested manylinux1 explicitly (i.e. we need to fail/skip pypy).
Add windows support
- Find a reference how a native pypy wheel on windows should look like: python_Levenshtein-0.10.2-pp38-pypy38_pp73-win_amd64.whl contains Levenshtein.pypy38-pp73-win_amd64.pyd: https://paste.rs/ZAJ
Test on mac (done by @messense, it works)
~~Skip pypy wheels when uploading~~
Autodetect pypy binaries, but only for pyo3
Look for a better way than using SOABI to determine the native extensions filename since that value isn't define on windows for both cpython and ~~pypy~~. SOABI is defined on windows for at least pypy 3.7 and later.
#460

I got here from a failing build of pywinpty when rebuilding for PyPy3.7 and windows. The failure is ERROR: pywinpty-1.1.3-pp3pp73-pypy3_pp73-win_amd64.whl is not a supported wheel on this platform. (link may dissapear in a week or so). The project uses maturin build --release -i pypy3.7.exe.

Bottom line: The wheel name should end with -pp37-pypy37_pp73-win_amd64.whl not -pp3pp73-pypy3_pp73-win_amd64.whl. Any ideas where the extra pp3 at the beninning is coming from?

Some additional comments:

Add some automated testing, ideally on ci as pyo3 does:
github actions setup-pythonv2 supports pypy
Look for a better way than using SOABI to determine the native extensions filename since that value isn't define on windows for both cpython and pypy:
I think sysconfig.get_config_var('SOABI') is the canonical way to do this. What versions of python do not support it? You could get some inspriation from the pypa/wheel source code

Bottom line: The wheel name should end with -pp37-pypy37_pp73-win_amd64.whl not -pp3pp73-pypy3_pp73-win_amd64.whl. Any ideas where the extra pp3 at the beninning is coming from?

maturin/src/python_interpreter.rs

Line 369 in cb839bd

"pp{major}{minor}-pypy{major}{minor}_{abi_tag}-{platform}",

It comes from here.

Got it thanks: that was changed for version 0.11.3, and the feedstock is pinning to <=0.9.1

All done.