py3: make check-py3-compat.py load modules in standard manner
Otherwise no code transformation would be applied to the modules which are
imported only by imp.load_module().
This change means modules are imported from PYTHONPATH, not from the paths
given by command arguments. This isn't always correct, but seems acceptable.
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors