Document and maintain Python pre-releases

Hi team :wave:

I suspect that some maintainers would appreciate an easily installable version of Python pre-releases to demonstrate how their project works with a prerelease's new feature.

As I understand the process of creating a pre-release so far:

- Prereleases are found in named branches on the python-feedstock repo
- The recipe's [`meta.yaml`](https://github.com/conda-forge/python-feedstock/blob/pre-3.8/recipe/meta.yaml) contains configuration for the release version number and a label to indicate it is a pre-release.

I'm not clear on:

- the series of patches in the `meta.yaml` file
- how to render/rerender a pre-release branch so the label applies (does it do it automagically with the existing commands?)

If folks see value in this, I would be happy to iterate the details here and then document it. Thanks!