Restrict gevent test on Windows to run on 3.6

tools/run_tests/run_tests.py

@@ -862,7 +862,12 @@ class PythonLanguage(object):
 
         if args.compiler == 'default':
             if os.name == 'nt':
-                return (python38_config,)
+                if args.iomgr_platform == 'gevent':
+                    # TODO(https://github.com/grpc/grpc/issues/23784) allow
+                    # gevent to run on later version once issue solved.
+                    return (python36_config,)
+                else:
+                    return (python38_config,)
             else:
                 if args.iomgr_platform == 'asyncio':
                     return (python36_config, python38_config)