Python 3.7 support

[PottyZA]

Hi!

I apologise if this doesn't follow a set issue template, as the link to that in the README appears to be broken.

I've been looking at using your Braze client in a project that I'm working on. This project is currently still on Python 3.7 for compatibility reasons (with plans to upgrade some time in the near future).

Would it be possible to drop the setup Python version requirement to Python 3.7?

I have experimented with this change already on a fork of this repository and it appears to work as-is (all the tests pass), but preferably I'd like to work off of the upstream repository instead of a custom fork that I'd have to maintain.