Add Python 3.7 support

[vtitor]

Add Python 3.7 support as soon as it is supported by Travis CI.
See the upstream issue travis-ci/travis-ci#9815.

[MapleCCC]

The upstream issue has been closed. Maybe it's time to add Python 3.x support? Also I would recommend to drop Python 2.x support in view of that Python 2.7 reached End-of-Life in Jan. 1st 2020. Some code could be rewritten to get rid of Python 2.x style, e.g. no need to inherit from object anymore in Python 3.x.

[case]

Looks like this is happening in #35!