Add Python 3.7

[scolby33]

I'd love to be able to build my docs for a Python 3.7-only project on RTD.

Since you're using pyenv from git, is it possible to add Python 3.7 to the rtd/build 3.x line of images, or will we have to wait until you switch to the 18.04-based image?

[agjohnson]

Yup! This would be a great addition. We were waiting for an actual release of 3.7 before adding to the image here. It's been out for a bit, but we've had very little bandwidth for this.

Targeting 4.1 image for this.

[webknjaz]

The related PR #73 is ready to be merged ;)

[vEpiphyte]

I added notes to #73 if there is a urgent need for 3.7 support for doc building using conda as a workaround for now.