pyplot: Fix exception in `_backend_selection` during import

if the new style introspection GObject python bindings are in use.  With
pygobject >= 3.13.4 the following:

    from gi.repository import GObject
    from matplotlib import pyplot

causes an exception to be raised:

> AttributeError: When using gi.repository you must not import static modules
> like "gobject". Please change all occurrences of "import gobject" to "from
> gi.repository import GObject". See:
> https://bugzilla.gnome.org/show_bug.cgi?id=709183

It is not valid to use both non-introspection based and introspection based
PyGObject in the same process.  Backend probing will `import gobject` (i.e. the
non-introspection bindings) if it sees that the 'gtk' module is loaded.
Unfortunately it wouldn't check if this was the pygi or old-style gtk module.
This commit adds this check avoiding the exception.

This check was added to PyGObject in [d704033][1]

[1]: https://git.gnome.org/browse/pygobject/commit/?id=d704033