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.