Merge pull request #5959 from royalharsh/master

DocFixit: Add instructions to install protobuf3 in python readme files

requirements.txt

@@ -1,7 +1,8 @@
 # GRPC Python setup requirements
+coverage>=4.0
+cython>=0.23
 enum34>=1.0.4
 futures>=2.2.0
-cython>=0.23
-coverage>=4.0
+protobuf>=3.0.0a3
 six>=1.10
-wheel>=0.29
+wheel>=0.29