test_tagopy.test_plat_name_ext and setting plat-name on non-pure wheels

[mattip]

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As fallout from #346, there was a change in behavior. Previously, a user could specify a plat-name and bdist_wheel would create a binary wheel with that plat-name. This is tested for in test_plat_name_ext. Before that PR the test passed, after it the test is skipped (the failed assert in get_tag is misinterpreted as an inability to compile).

Arguably, the previous behavior was ~unintentional~ intended but questionable since

• there is a comment that only pure wheels should be allowed to specify plat-name
• it is not clear how pip would install such a wheel
• there is a test to make sure it is possible.

As @carljm points out in this comment, it is a change in behaviour.

[agronholm]

I still want to hear his use case to determine if we should fix this or not.

[carljm]

Described the use case in https://github.com/pypa/wheel/pull/346#issuecomment-682318511

[agronholm]

I think this was fixed by #375 but I forgot to mention it in the release notes. Can you confirm?

[mattip]

I guess it would be nice to add a blurb like "Fixed regression in v35 where --plat-name was ignored for c-extension packages. Also fixed faulty test for this flag"