bpo-10945: Drop support for bdist_wininst on non-Windows systems (GH-14506)

bpo-10945: Drop support for bdist_wininst on non-Windows systems (GH-14506)

bdist_wininst depends on MBCS codec, unavailable on non-Windows,
and bdist_wininst have not worked since at least Python 3.2, possibly
never on Python 3.

Here we document that bdist_wininst is only supported on Windows,
and we mark it unsupported otherwise to skip tests.

Distributors of Python 3 can now safely drop the bdist_wininst .exe files
without the need to skip bdist_wininst related tests.