test_customize_compiler_with_msvc_python: skip for cygwin

Mingw32CCompiler() checks that the default compiler isn't cygwin,
so it can't be used under cygwin, so skip it there.

distutils/tests/test_mingwccompiler.py

@@ -45,6 +45,7 @@ def test_cygwincc_error(self, monkeypatch):
         with pytest.raises(CCompilerError):
             distutils.cygwinccompiler.Mingw32CCompiler()
 
+    @pytest.mark.skipif('sys.platform == "cygwin"')
     def test_customize_compiler_with_msvc_python(self):
         from distutils.cygwinccompiler import Mingw32CCompiler