How to enforce a particular Python version during port build

[Ralph Seichter]

* Blair Zajac:

> Is this port only for you and you’ll not commit it to the Git repo?

As of now, I am only trying that particular build for myself.

> Which package is this?

I am trying to build MongoDB 4.2.0 by modifying the existing Portfile
(that's version 4.0.12). According to the docs[1], building MongoDB
requires Python 3.7. There's obviously more to it than just changing
'depends_build'.

I can of course build MongoDB manually, but where's the fun in that? ;-)

-Ralph