Python 3.7 support

[mfojtak]

Is your feature request related to a problem? Please describe.
Python 3.8 is still too fresh and not supported by Anaconda.

Describe the solution you'd like
The library to be available on pip for Python 3.7.

[dbanty]

Sure, I poked around and I think the feature losses needed are worth supporting more people.

0.4.1 should be out soon with Python 3.7 support. Please try it out and make sure the generated client also works with 3.7. I've added 3.7 to the CI environment but I'd like you to confirm before we close this out.

[dbanty]

Going to assume this is all set since I haven't heard any complaints. We can always open new issues if there are bugs found.