Adding py38 subports? Mind the dependencies

[Ryan Schmidt]

There have been a lot of commits adding py38 subports to python module ports, when those ports' dependencies don't yet have py38 subports. Of course, this will fail to build.

If you're going to add a python version to a port, please make sure its dependencies exist.

This is a more specific case of the general rule: Please make sure your changes build successfully on your system before you commit them. Yes, for python module updates, that means trying to install all of the subports.