Repository: googleapis/google-cloud-python

Issue:
PubSub protobuf dependency requirements incorrect
I think the minimum version for the `protobuf` package dependency is not correct.  google-cloud-python has the version requirements as protobuf>=3.0.0, but it fails to import when using version up to and including protobuf==3.3.0.  I'm not sure what the exact correct version is, but the last version of google-cloud-pubsub to work with protobuf==3.3.0 is google-cloud-pubsub==0.35.4.  I believe after this commit (https://github.com/GoogleCloudPlatform/google-cloud-python/commit/371333a51165e99d4d02876b1ef133618485b6fc#diff-29280288794caf553b0b008084a0e854), a protobuf version >3.3.0 is required:

Python version
```
$ python --version
Python 2.7.15rc1
```

Package versions:
```
$ pip list | grep -E '(cloud|protobuf)'
google-cloud-core                  0.28.1      
google-cloud-datastore             1.7.0       
google-cloud-pubsub                0.38.0      
google-cloud-storage               1.12.0      
protobuf                           3.3.0 
```

Getting a stack track just importing pubsub (in ipython here)
```
In [1]: from google.cloud import pubsub
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-1-8fba37b708ad> in <module>()
----> 1 from google.cloud import pubsub

/home/aaronpeterson/.local/share/virtualenvs/turbinia-docs-oOHvuNoj/local/lib/python2.7/site-packages/google/cloud/pubsub.py in <module>()
     17 from __future__ import absolute_import
     18 
---> 19 from google.cloud.pubsub_v1 import PublisherClient
     20 from google.cloud.pubsub_v1 import SubscriberClient
     21 from google.cloud.pubsub_v1 import types

/home/aaronpeterson/.local/share/virtualenvs/turbinia-docs-oOHvuNoj/local/lib/python2.7/site-packages/google/cloud/pubsub_v1/__init__.py in <module>()
     15 from __future__ import absolute_import
     16 
---> 17 from google.cloud.pubsub_v1 import types
     18 from google.cloud.pubsub_v1 import publisher
     19 from google.cloud.pubsub_v1 import subscriber

/home/aaronpeterson/.local/share/virtualenvs/turbinia-docs-oOHvuNoj/local/lib/python2.7/site-packages/google/cloud/pubsub_v1/types.py in <module>()
     28 
     29 from google.api_core.protobuf_helpers import get_messages
---> 30 from google.cloud.pubsub_v1.proto import pubsub_pb2
     31 
     32 

/home/aaronpeterson/.local/share/virtualenvs/turbinia-docs-oOHvuNoj/local/lib/python2.7/site-packages/google/cloud/pubsub_v1/proto/pubsub_pb2.py in <module>()
     45       message_type=None, enum_type=None, containing_type=None,
     46       is_extension=False, extension_scope=None,
---> 47       options=None, file=DESCRIPTOR),
     48   ],
     49   extensions=[

TypeError: __new__() got an unexpected keyword argument 'file'
```

Snipped the pubsub section from pipdeptree output showing the protobuf requirement is >=3.0.0:
```
 - google-cloud-pubsub [required: Any, installed: 0.38.0]
    - enum34 [required: Any, installed: 1.1.6]
    - google-api-core [required: >=1.1.0,<2.0.0dev, installed: 1.4.0]
      - futures [required: >=3.2.0, installed: 3.2.0]
      - google-auth [required: >=0.4.0,<2.0.0dev, installed: 1.5.1]
        - cachetools [required: >=2.0.0, installed: 2.1.0]
        - pyasn1-modules [required: >=0.2.1, installed: 0.2.2]
          - pyasn1 [required: >=0.4.1,<0.5.0, installed: 0.4.4]
        - rsa [required: >=3.1.4, installed: 4.0]
          - pyasn1 [required: >=0.1.3, installed: 0.4.4]
        - six [required: >=1.9.0, installed: 1.11.0]
      - googleapis-common-protos [required: >=1.5.3,<2.0dev, installed: 1.5.3]
        - protobuf [required: >=3.0.0, installed: 3.3.0]
          - setuptools [required: Any, installed: 40.4.3]
          - six [required: >=1.9, installed: 1.11.0]
      - protobuf [required: >=3.0.0, installed: 3.3.0]
        - setuptools [required: Any, installed: 40.4.3]
        - six [required: >=1.9, installed: 1.11.0]
```


Discussion:

Resolving patch:
diff --git a/pubsub/setup.py b/pubsub/setup.py
--- a/pubsub/setup.py
+++ b/pubsub/setup.py
@@ -29,7 +29,7 @@
 # 'Development Status :: 5 - Production/Stable'
 release_status = 'Development Status :: 4 - Beta'
 dependencies = [
-    'google-api-core[grpc] >= 1.1.0, < 2.0.0dev',
+    'google-api-core[grpc] >= 1.4.1, < 2.0.0dev',
     'grpc-google-iam-v1 >= 0.11.1, < 0.12dev',
     'enum34; python_version < "3.4"',
 ]


Test patch: