fix: pin protobuf to < 3.18.0 (#277)

Fixes #276. This PR is being made to the v1 branch and will be released manually in 1.31.3.

Protobuf 3.18.0 removed support for Python 2.7 and 3.5 https://github.com/protocolbuffers/protobuf/releases/tag/v3.18.0. Because the protobuf library does not use python_requires, pip will try to install it even on Python 2.7. A new release of google-api-core should result in google-cloud-* library users getting the last version of protobuf they can use.

The latest versions of Cloud libraries (except for google-cloud-storage) have moved on to python >= 3.6. However, users google-cloud-storage and older Cloud libraries may be impacted.

Also see protocolbuffers/protobuf#8984

Author: busunkim96

setup.py

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