import inspect (protocolbuffers#3507)

Inspect is used five times on line 65-74 without being imported.

Author: cclauss

python/stubout.py

@@ -17,6 +17,9 @@
 # This file is used for testing.  The original is at:
 #   http://code.google.com/p/pymox/
 
+import inspect
+
+
 class StubOutForTesting:
   """Sample Usage:
      You want os.path.exists() to always return true during testing.