tags._generic_abi should use only the first part of the SOABI

[mattip]

In _generic_abi() (which is used on non-CPython), the abi tag is equivalent to the SOABI sysconfig value. But this is "cpython-38-x86_64-linux-gnu" on CPython. The equivalent code path for CPython goes to _cpython_abis() which does not use SOABI at all. Currently, PyPy must keep its SOABI to "pypy38-pp73" in order to allow pip to function. The EXTSUFFIX is "".pypy38-pp73-x86_64-linux-gnu.so". This was fixed for wheel in pypa/wheel#372 and should be fixed here as well.

It seems CMake uses the SOABI, so building projects with PyPy there requires work-arounds. Here is the PyPy issue https://foss.heptapod.net/pypy/pypy/-/issues/3816

[mattip]

thinking about how to test this: the same result should be produced by _cpython_abis() and _generic_abis()

[brettcannon]

Some other details can be found in #606 which was failed simultaneously.

[brettcannon]

Welcome to the historical baggage that is wheel tag generation. 😉 Not saying we can't consider changing it, but this wasn't by design as much as organic growth when I collected all the various implementations together and this bifurcation of logic already existed.