Rolling readthedocs version back to 3.7.
Rolling readthedocs version back to 3.7.

From a failed build [1]

> Problem in your project's configuration. Invalid "python.version":
> expected one of (2, 2.7, 3, 3.5, 3.6, 3.7, pypy3.5), got 3.8

[1]: https://readthedocs.org/projects/bezier/builds/10221076/

dhermes/bezier,dhermes/bezier,dhermes/bezier