* 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).