Change Python code to use new_reference

Change Python code to use new_reference

This changes a few spots in the Python code to use new_reference
rather than the manual incref+constructor that was previously being
done.

ChangeLog
2018-04-30  Tom Tromey  <tom@tromey.com>

	* varobj.c (varobj_set_visualizer): Use new_reference.
	* python/python.c (gdbpy_decode_line): Use new_reference.
	* python/py-cmd.c (cmdpy_function, cmdpy_completer_helper): Use
	new_reference.