Add official Dlint support for all currently supported Python versions

[mschwager]

Dlint initially targeted Python 2.7 and 3.6. We should officially support all versions of Python that have active development cycles.

"Official support" for Dlint means we'll run our tests on that version, and include that version in our classifiers in setup.py. This task depends on having TravisCI set up to run with the appropriate Python versions. Once the tests pass on a particular version we can include its classifier and "officially support" it.

More explicitly, this is all versions of Python that are not end-of-life'd and have had a final release cut, currently: 2.7, 3.5, 3.6, 3.7.

[mschwager]

Closed with the release of version 0.6.0.