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.
• Allow building for non-abi3 pypy wheels when the abi3 feature is enabled #460