make temporary read-only files writable, so rmtree can remove them (#21128)

Lib/test/test_argparse.py

@@ -47,6 +47,9 @@ class TempDirMixin(object):
 
     def tearDown(self):
         os.chdir(self.old_dir)
+        for root, dirs, files in os.walk(self.temp_dir, topdown=False):
+            for name in files:
+                os.chmod(os.path.join(self.temp_dir, name), stat.S_IWRITE)
         shutil.rmtree(self.temp_dir, True)
 
     def create_readonly_file(self, filename):