Don't use os.normcase when the result we are expecting is None.

This worked fine on linux but fails on Windows.  That may or may
not be a but in normcase.

Lib/test/test_inspect.py

@@ -278,7 +278,7 @@ class TestRetrievingSourceCode(GetSourceBase):
         self.assertEqual(normcase(inspect.getsourcefile(git.abuse)), modfile)
         fn = "_non_existing_filename_used_for_sourcefile_test.py"
         co = compile("None", fn, "exec")
-        self.assertEqual(normcase(inspect.getsourcefile(co)), None)
+        self.assertEqual(inspect.getsourcefile(co), None)
         linecache.cache[co.co_filename] = (1, None, "None", co.co_filename)
         self.assertEqual(normcase(inspect.getsourcefile(co)), fn)