fix(deps): allow protobuf >=3.18.0 for python 3 users (#305)

https://github.com/protocolbuffers/protobuf/releases/tag/v3.18.0 dropped Python 2 support. Python 3 users can continue to use the latest version of protobuf.

Author: busunkim96

setup.py

@@ -30,7 +30,8 @@
 release_status = "Development Status :: 5 - Production/Stable"
 dependencies = [
     "googleapis-common-protos >= 1.6.0, < 2.0dev",
-    "protobuf >= 3.12.0, < 3.18.0",
+    'protobuf >= 3.12.0, < 3.18.0; python_version < "3"',
+    'protobuf >= 3.12.0; python_version > "3"',
     "google-auth >= 1.25.0, < 2.0dev",
     "requests >= 2.18.0, < 3.0.0dev",
     "setuptools >= 40.3.0",