Forbid protobuf>=4.0

requirements.txt

@@ -8,7 +8,7 @@ sortedcontainers
 uvloop>=0.14.0
 grpcio>=1.33.2
 grpcio-tools>=1.33.2
-protobuf>=3.12.2
+protobuf>=3.12.2,<4.0
 configargparse>=1.2.3
 multiaddr>=0.0.9
 pymultihash>=0.8.2