Fix issue16300: addressing the buildbot failures on windows

Lib/test/test_urllib.py

@@ -275,10 +275,9 @@ Content-Type: text/html; charset=iso-8859-1
         self.assertTrue(e.exception.filename)
         self.assertTrue(e.exception.reason)
 
-    @unittest.skipIf(sys.platform == "win32", "Skip on Windows: issue16300")
     def test_file_notexists(self):
         fd, tmp_file = tempfile.mkstemp()
-        tmp_fileurl = 'file://localhost' + tmp_file
+        tmp_fileurl = 'file://localhost/' + tmp_file.replace(os.path.sep, '/')
 
         self.assertTrue(os.path.exists(tmp_file))
         with urlopen(tmp_fileurl) as fobj: