How to enforce a particular Python version during port build

[Ralph Seichter]

* Clemens Lang:

> Re-iterating on that, the problem is likely SCons [...]

I appreciate all your guys' input.

After some much-needed sleep, I have decided that I do find having a
Portfile for MongoDB 4.2.0 on macOS an interesting challenge, but given
my time constraints I'll stick with the official binaries for now (see
https://www.mongodb.com/download-center/community). I am guessing it was
mostly my dark, Gentoo Linux side that was pushing me towards building
from the sources. ;-)

-Ralph