fix(deps): require protobuf>= 3.12.0, <4.0.0dev (#387)

Author: parthea

setup.py

@@ -31,7 +31,7 @@
 dependencies = [
     "googleapis-common-protos >= 1.6.0, < 2.0dev",
     'protobuf >= 3.12.0, < 3.18.0; python_version < "3"',
-    'protobuf >= 3.12.0; python_version > "3"',
+    'protobuf >= 3.12.0, < 4.0.0dev; python_version > "3"',
     "google-auth >= 1.25.0, < 2.0dev",
     "requests >= 2.18.0, < 3.0.0dev",
     "setuptools >= 40.3.0",