pkg/54791: python version choosing logic doesn't test what's installed

[dholland%eecs.harvard.edu@localhost]

>Number:         54791
>Category:       pkg
>Synopsis:       python version choosing logic doesn't test what's installed
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    pkg-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Dec 21 08:55:00 +0000 2019
>Originator:     David Holland
>Release:        NetBSD-8 (oldish), pkgsrc 20191220
>Organization:
>Environment:
System: n/a
Architecture: i386
Machine: i386
>Description:

The logic in pyversion.mk that picks a Python version to depend on
*says*
   # prefer an already installed version, in order of "accepted"
but it doesn't actually do it.

The consequence of this is that if you set PYTHON_VERSION_DEFAULT back
(in this case setting it to 27 is necessary at least for a while for
migration reasons)... even if you install say python37, a lot of
things go and build bleeding-edge python instead, unexpectedly, and
then the right modules aren't there and other parts of the build fail.

I think this is a regression, in that I'm pretty sure it used to work
as described. But it's been a while since it mattered.

>How-To-Repeat:

install python27 and python37
cd devel/glib2-tools	(or quite a few other things)
env PYTHON_VERSION_DEFAULT=27 make
watch it go build python38

(then confirm in pyversion.mk)

>Fix: