Recommend manylinux wheels in the Error 400 response for "linux" package uploads

[ncoghlan]

What's the problem this feature will solve? Providing better guidance to users when they fail to upload a wheel with a "linux" platform tag

Currently these users get an opaque error message like "HTTPError: 400 Client Error: Binary wheel 'treap-1.39-cp36-cp36m-linux_x86_64.whl' has an unsupported platform tag 'linux_x86_64'. for url: https://test.pypi.org/legacy/"

Describe the solution you'd like

• a tailored 400 response for "linux_x86_64" and "linux_i386" that points to a page introducing users to manylinux, auditwheel, and related tools like cibuildwheel and conda-press.

Additional context

This feature request was prompted by Dan Stromberg's issue report in https://mail.python.org/archives/list/distutils-sig@python.org/message/RSJ57E343I7NVB6QDFFBD4QNTHRVAYOT/

Fixing this would likely require at least an initial attempt at addressing https://github.com/pypa/packaging.python.org/issues/637, as there isn't currently a good end user facing page for a custom error page to link to (the manylinux and auditwheel project pages aren't suitable)

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[di]

I added the good first issue label to this issue.

[di]

Blocked on https://github.com/pypa/packaging.python.org/issues/637.