feature: Add Python 3.7 support

[ajaykarpur]

Description of changes:

• Update buildspecs to test against py37
• Update Python version in setup.py
• Update tox envlist

Testing done:
Ran unit tests locally and testing with the PR build system.

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

General

• I have read the CONTRIBUTING doc
• I used the commit message format described in CONTRIBUTING
• I have used the regional endpoint when creating S3 and/or STS clients (if appropriate)
• I have updated any necessary documentation, including READMEs

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have checked that my tests are not configured for a specific region or account (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.