google-cloud-pubsub==0.28.4 requires google-cloud-core[grpc] > 0.27.1, <0.28dev. I can see from the core library's setuptools build that the grpc extra is declared; however, setuptools does not seem to be able to locate it from the published PyPi package for whatever reason:

$ docker run python:3 bash -c "pip install google-cloud-core[grpc]"  # python:2 no different
Collecting google-cloud-core[grpc]
  Downloading google_cloud_core-0.27.1-py2.py3-none-any.whl (50kB)
  google-cloud-core 0.27.1 does not provide the extra 'grpc'
...
Successfully installed cachetools-2.0.1 certifi-2017.7.27.1 chardet-3.0.4 google-auth-1.1.1 google-cloud-core-0.27.1 googleapis-common-protos-1.5.3 idna-2.6 protobuf-3.4.0 pyasn1-0.3.7 pyasn1-modules-0.1.5 requests-2.18.4 rsa-3.4.2 six-1.11.0 urllib3-1.22

When installing via pip this is not really an issue, so long as grpcio is included somewhere in the dependency tree, as pip will happily ignore it. Setuptools itself will not, however, and including google-cloud-pubsub==0.28.4 as a requirement in a setuptools build is currently impossible, so far as I can tell.