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.

command/bdist_wininst.py

@@ -55,6 +55,9 @@ class bdist_wininst(Command):
     boolean_options = ['keep-temp', 'no-target-compile', 'no-target-optimize',
                        'skip-build']
 
+    # bpo-10945: bdist_wininst requires mbcs encoding only available on Windows
+    _unsupported = (sys.platform != "win32")
+
     def initialize_options(self):
         self.bdist_dir = None
         self.plat_name = None