this PR allows you to install python via `asdf install python 3.10` or
`asdf install python 3`. It will then install the latest version
starting with that prefix.
It will add an alias in ~/.asdf/installs/python so you can have a
.tool-versions file like the following:
```
python 3.10
```
`python-build` supports a `-p` option to patch Python before installing. In
particular, I use this to apply a patch for OS X Big Sur before installing
older versions of Python.
I'm not super-wedded to this particular implementation: opening this PR for
more discussion.
