Add Python 3.7 support

[joeyespo]

Note that this required upgrading Travis CI's dist to xenial.

References:

• https://docs.travis-ci.com/user/reference/xenial/
• https://docs.travis-ci.com/user/languages/python/#python-37-and-higher
• https://docs.travis-ci.com/user/reference/overview/#deprecated-virtualization-environments