add support for Python 3.7

[Jenselme]

Original patch by Miro Hrončok. See https://bugzilla.redhat.com/show_bug.cgi\?id\=1605884

[twolfson]

Unfortunately Travis CI doesn't yet support Python 3.7 so the tests are failing =/ I'm happy to land this but I think we'll need to remove 3.7 from .travis.yml first

[Jenselme]

I think we can wait a bit for Python 3.7 on travis. It should come soon.

[twolfson]

Cool, works for me. Leaving this PR open until then

[dotlambda]

Any updates?

[twolfson]

In theory it should work as Travis CI has 3.7 support, https://docs.travis-ci.com/user/languages/python/, but I'm still seeing errors. It looks like __dir is being picked up as a class-level variable due to its __ prefix and the 3.7 build is failing to compile PyYAML

@dotlambda If this is high priority for you, then I'm happy to accept a PR which only updates the library to stop using __dir__ (e.g. moves to _dir) and passes existing Python versions

[Jenselme]

@twolfson @dotlambda I just updated this PR and travis build passed!

[twolfson]

Great, thanks @Jenselme! Landing this now

[twolfson]

This has been landed in 1.3.0 and pushed to PyPI. Thanks again!

[Jenselme]

You're welcomed! And thanks for your package!