Forbid protobuf 4.x in requirements (#508)

forbid protobuf 4.x for now

requirements.txt

@@ -7,7 +7,7 @@ msgpack>=0.5.6
 sortedcontainers
 uvloop>=0.14.0
 grpcio-tools>=1.33.2
-protobuf>=3.12.2
+protobuf>=3.12.2,<4.0.0
 configargparse>=1.2.3
 multiaddr>=0.0.9
 pymultihash>=0.8.2