Removing Python 3.8 and 3.9 support

[Thomas Klausner <wiz%NetBSD.org@localhost>]

Hi!

I'd like to remove 38 and 39 from the default-acceptable list for
Python modules.

The base python38 and python39 packages will remain in the pkgsrc
tree, just, by default, no modules will be built for them.

The reason is that many upstreams have already dropped support for
Python 3.8 and some important ones have started doing it for 3.9.

Also, I'm not aware of any bigger issues switching from 3.8 or 3.9 to
3.10 or newer.

Comments?
 Thomas