Today, our daily gcloud ml-engine job has been broken. The problem may come from the changes in the module grpc-google-iam-v1. It seems that you did an update for the module grpc-google-iam-v1 from 0.11.1 (?) to 0.11.3 on the server-side.
I compared the "broken" version with the "working" version. I realized that the only diff is in the module grpc-google-iam-v1.

Python version: Python 2.7

Google-Cloud version:

google-cloud==0.24.0
google-cloud-storage==1.2.0
grpc-google-iam-v1==0.11.3

It has been broken because the pip tried to install grpc-google-iam-v1==0.11.3 which did not have the attribute RegisterServiceDescriptor in the object SymbolDatabase

Stacktrace:
Traceback (most recent call last): File "/usr/local/bin/pip", line 5, in <module> from pkg_resources import load_entry_point File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 3074, in <module> @_call_aside File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 3058, in _call_aside f(*args, **kwargs) File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 3102, in _initialize_master_working_set for dist in working_set File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 3102, in <genexpr> for dist in working_set File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 2633, in activate declare_namespace(pkg) File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 2191, in declare_namespace _handle_ns(packageName, path_item) File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 2126, in _handle_ns loader.load_module(packageName) File "/usr/lib/python2.7/pkgutil.py", line 246, in load_module mod = imp.load_module(fullname, self.file, self.filename, self.etc) File "/root/.local/lib/python2.7/site-packages/google/cloud/pubsub/__init__.py", line 30, in <module> from google.cloud.pubsub.client import Client File "/root/.local/lib/python2.7/site-packages/google/cloud/pubsub/client.py", line 28, in <module> from google.cloud.pubsub._gax import _PublisherAPI as GAXPublisherAPI File "/root/.local/lib/python2.7/site-packages/google/cloud/pubsub/_gax.py", line 19, in <module> from google.cloud.gapic.pubsub.v1.publisher_client import PublisherClient File "/root/.local/lib/python2.7/site-packages/google/cloud/gapic/pubsub/v1/publisher_client.py", line 37, in <module> from google.iam.v1 import iam_policy_pb2 File "/root/.local/lib/python2.7/site-packages/google/iam/v1/iam_policy_pb2.py", line 296, in <module> _sym_db.RegisterServiceDescriptor(_IAMPOLICY) AttributeError: 'SymbolDatabase' object has no attribute 'RegisterServiceDescriptor'

Steps to reproduce:
It's very easy to reproduce:
Cause of #3736, we have to freeze the following version:

google-cloud == 0.24.0
google-cloud-storage == 1.2.0

ps: I did not test the latest version yet but at least the grpc-google-iam-v1==0.11.3 is not compatible with these versions google-cloud

Workaround: freeze the version grpc-google-iam-v1==0.11.1