fix(deps): require protobuf<4.0.0 on v2 branch (#356)

Author: parthea

setup.py

@@ -30,8 +30,9 @@
 release_status = "Development Status :: 5 - Production/Stable"
 dependencies = (
     "google-api-core[grpc] >= 1.31.5, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.0",
-    "proto-plus >= 1.4.0",
-    "libcst >= 0.2.5",
+    "proto-plus >= 1.4.0, <2.0.0dev",
+    "libcst >= 0.2.5, <1.0.0dev",
+    "protobuf<4.0.0dev",
 )
 extras = {}