PGI bindings for gtk3
---------------------

The GTK3 backends can now use PGI_ instead of PyGObject_.  PGI is a fairly
incomplete binding for GObject, thus its use is not recommended; its main
benefit is its availability on Travis (thus allowing CI testing for the gtk3agg
and gtk3cairo backends).

The binding selection rules are as follows:
- if ``gi`` has already been imported, use it; else
- if ``pgi`` has already been imported, use it; else
- if ``gi`` can be imported, use it; else
- if ``pgi`` can be imported, use it; else
- error out.

Thus, to force usage of PGI when both bindings are installed, import it first.

.. _PGI: https://pgi.readthedocs.io/en/latest/
.. _PyGObject: http://pygobject.readthedocs.io/en/latest/#