* lang/python/examples/howto/groups.py: subprocess update
* lang/python/examples/howto/export-secret-keys.py: subprocess update
Both of these try the nice and easy method of getting the subprocess
output available in Python 3, but will fall back to the older Popen
method if it doesn't work. Essentially this is to be a little nicer
to Python 2.7.15 (even though the examples are filled with warnings
that py2 support is not guaranteed with the examples).
* Fixed the groups.py script so it really does what is described (the
old code had the same result for groups, group_lines and
group_lists).
* Updated the corresponding example in the doc to match.
* Updated usage so it only references importing the final list of
lists produced. Trying to use some of the mid-points can have
unpredictable results (this is part of the problem with work
arounds).
* Added groups selection work around code.
* Intended for use as a module to be imported by other scripts,
usually with "from groups import group_lists" or "from groups import
group_lines" or similar.
