Add get_musl_major_minor from PEP 656.

[dHannasch]

Since packaging supports PEP 656 wheels (pypa/packaging#411), I think all that is necessary to allow pip to install them is to have the platform check pick up the platform tag.

For example, right now rpy2-3.4.5-cp39-cp39-musllinux_1_2_x86_64.whl (tag cp39-cp39-musllinux_1_2_x86_64 @ 140319066709760) seems ready to install except for the fact that, on an actual musllinux system, get_supported() returns only [<cp39-cp39-linux_x86_64 @ 140319067309696>, <cp39-abi3-linux_x86_64 @ 140319066714880>, <cp39-none-linux_x86_64 @ 140319066715456>, <cp38-abi3-linux_x86_64 @ 140319066714240>, <cp37-abi3-linux_x86_64 @ 140319066715712>, <cp36-abi3-linux_x86_64 @ 140319066715968>, <cp35-abi3-linux_x86_64 @ 140319066716224>, <cp34-abi3-linux_x86_64 @ 140319066716480>, <cp33-abi3-linux_x86_64 @ 140319066716736>, <cp32-abi3-linux_x86_64 @ 140319066716992>, <py39-none-linux_x86_64 @ 140319066714304>, <py3-none-linux_x86_64 @ 140319066714688>, <py38-none-linux_x86_64 @ 140319066717696>, <py37-none-linux_x86_64 @ 140319066717952>, <py36-none-linux_x86_64 @ 140319066726464>, <py35-none-linux_x86_64 @ 140319066726720>, <py34-none-linux_x86_64 @ 140319066726976>, <py33-none-linux_x86_64 @ 140319066727232>, <py32-none-linux_x86_64 @ 140319066727488>, <py31-none-linux_x86_64 @ 140319066727744>, <py30-none-linux_x86_64 @ 140319066728000>, <cp39-none-any @ 140319066728512>, <py39-none-any @ 140319066728256>, <py3-none-any @ 140319066728768>, <py38-none-any @ 140319066729024>, <py37-none-any @ 140319066729280>, <py36-none-any @ 140319066729536>, <py35-none-any @ 140319066729792>, <py34-none-any @ 140319066730048>, <py33-none-any @ 140319066730304>, <py32-none-any @ 140319066730624>, <py31-none-any @ 140319066730880>, <py30-none-any @ 140319066731136>].

By itself, this doesn't change any behavior. I haven't figured out the right place to slot into the various additions to the supported list. But if I'm on the right track at all, then we'll need the get_musl_major_minor function from PEP 656 in compatibility_tags.py, and if not, then I'm on completely the wrong track.

[uranusjr]

Where does pip need this? pip did not need to know about parsing glibc versions for manylinux, so I fail to see why it needs to know about musl versions for musllinux. AFAICT pip can simply relies on packaging.tags.get_tags() to do everything it needs.

[dHannasch]

pip simply relies on packaging.tags.get_tags() to do everything it needs.

Thank you! It turns out upgrading packaging (#10089) was all that was needed. I have that running and installing musllinux_1_2_x86_64 wheels in https://hub.docker.com/orgs/pythonpackagesonalpine/repositories right now. (Which of course means we're closing in on the day when https://hub.docker.com/orgs/pythonpackagesonalpine/repositories will be retired in favor of just installing wheels.)