proposed python change: removing python 3.7

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

Hi!

Python 3.7 is on its way out, modules are starting to desupport it.
The 3.x versions are generally quite easy to switch between, and I
don't think the effort in supporting Python 3.7 is well-spent.

I therefore suggest removing this version from pkgsrc.
 Thomas