Make sure >3 Python version triggers a failure

src/python/grpcio/support.py

@@ -37,7 +37,12 @@ Could not find <Python.h>. This could mean the following:
   * You're on Windows and your Python installation was somehow corrupted
     (check your environment variables or try re-installing?)
 """
-PYTHON_REPRESENTATION = 'python' if sys.version_info[0] == 2 else 'python3'
+if sys.version_info[0] == 2:
+    PYTHON_REPRESENTATION = 'python'
+elif sys.version_info[0] == 3:
+    PYTHON_REPRESENTATION = 'python3'
+else:
+    raise NotImplementedError('Unsupported Python version: %s' % sys.version)
 
 C_CHECKS = {
     C_PYTHON_DEV: