skip test run if running with pypy

don't run cygrpc_test.TypeSmokeTest.testCallCredentialsFromPluginUpdown
if the interpreter is PyPy

src/python/grpcio_tests/tests/unit/_cython/cygrpc_test.py

@@ -30,6 +30,7 @@
 import time
 import threading
 import unittest
+import platform
 
 from grpc._cython import cygrpc
 from tests.unit._cython import test_utilities
@@ -113,6 +114,9 @@ class TypeSmokeTest(unittest.TestCase):
         lambda ignored_a, ignored_b: None, b'')
     del plugin
 
+  @unittest.skipIf(
+    platform.python_implementation() == "PyPy",
+    'TODO(issue 7672): figure out why this fails on PyPy')
   def testCallCredentialsFromPluginUpDown(self):
     plugin = cygrpc.CredentialsMetadataPlugin(_metadata_plugin_callback, b'')
     call_credentials = cygrpc.call_credentials_metadata_plugin(plugin)