Upgrade worker to support Python3.7

[asavaritayal]

Now that it's out of RC.

TBD - Investigate implications for the Azure SDK and other libraries that are only compatible with 3.6 so far.

[kapilt]

fwiw, we're doing builds/tests of azure client lib usage with 3.7, seems to work well. https://travis-ci.org/capitalone/cloud-custodian/jobs/409066825

[asavaritayal]

• Update CI to test for Python 3.7
• Updated underlying Docker image

[priyaananthasankar]

ML libraries need to get up to speed with Python 3.7 distros. 3.6 stability is preferred. Scikit already has issues and they have a roadmap.
scikit-learn/scikit-learn#11378
Will add more data points here.

[asavaritayal]

Thanks @priyaananthasankar .

Posting the highlights here from standup:

1. Changes that @elprans made yesterday make the language worker agnostic of whether the user is on 3.6 or 3.7. These changes will go out with the next release.
2. Additional work item required is to update the docker image to use 3.7 instead.

We'll hold back on #2 until python3.7 is more stable. Leaving the issue open for now.