python: selftesthelpers: Add possibility for planning tests for

python: selftesthelpers: Add possibility for planning tests for
'extra_python' (Python 3).

This change allows us to mark modules which are Python 3 compatible
with the keyword argument `py3_compatible`. Then, if building with
Python 3 is configured using --extra-python, `planpythontestsuite`
will plan an extra Python test for each marked module.

Signed-off-by: Lumir Balhar <lbalhar@redhat.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>