Test on Python37

[max-sixty]

What do you think about skipping conda tests for the moment @tswast ?

The test on 3.7 seems less controversial (probably should split these up, but both small PRs)

[stickler-ci]

The following files do not match the black styleguide:

• nox.py

[stickler-ci]

The following files do not match the black styleguide:

• nox.py

[max-sixty]

@tswast let me know if you recognize these ADC errors in the 3.7 build: https://travis-ci.org/max-sixty/pandas-gbq/jobs/422752133

Could they be related to us using sudo (which is required for 3.7 it seems)?

[tswast]

http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/?recursive=true from the Google Compute Enginemetadata service. Status: 404 Response

I think this is related to the fact that Travis CI runs on GCE but it locks down access to APIs. Maybe google-auth doesn't account for this case?

[tswast]

Let's

• Sync to master
• Make sure we include a Python 3.6 build, still.

I think syncing to master should pick up the fix for #226, which looks like the same reason that the Python 3.7 build was failing here.

[tswast]

Python 3.7 builds added in #232