pkg/51431: installing both python2 and python3 py- packages creates stale workdirs

[coypu%sdf.org@localhost]

>Number:         51431
>Category:       pkg
>Synopsis:       installing both python2 and python3 py- packages creates stale workdirs
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    pkg-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Aug 20 19:50:00 +0000 2016
>Originator:     coypu
>Release:        NetBSD 6.1.5, pkgsrc-2016Q3 (current now)
>Organization:
>Environment:
NetBSD 6.1.5 NetBSD 6.1.5 (GENERIC) amd64
>Description:
Using PYTHON_VERSION_DEFAULT=35 you will encounter this issue fairly quickly.

Package A can handle python3 and requires py-expat, you build py35-expat.
Package B requires python2 and py-expat too, but now you have a work
directory for python3, and it fails to build.

Will likely not be a "real" problem for some time, but it's possible that
in the future, python3 packages will not support python2.
>How-To-Repeat:

>Fix: