Transition PyPy to PEP 3149 compliant SOABI tags

[mattip]

It turns out PyPy's SOABI tag is not compliant with PEP 3149. Instead of containing both the ABI tag and the platform tag, PyPY's value returned for the SOABI query is only the ABI tag. For instance, on linux 64 CPython returns cpython-38-x86_64-linux-gnu but PyPy returns pypy36-pp73 where the correct value is pypy36-pp73-x86_64-linux-gnu.

I want to fix this on PyPy for our upcoming release, but then get_abi_tag will break. It correctly trims the platform tag off the SOABI value on line 94. Eventually we should get to a situation where line 94 works for both PyPy and CPython, but there needs to be a transition period based on either direct value parsing or some kind of version check.

Any thoughts? PyPy users are pretty good about updating, I think within a year we could drop any temporary shims we put in to work around this issue, under the assumption that by then anyone building a PyPy wheel will have upgraded to the latest version.

[agronholm]

Can you make a PR?