Remove old matplotlib.externals imports.

I guess nothing tests axis artist?
matplotlib · dopplershift · Sep 26, 2017 · Nov 4, 2016 · Mar 24, 2017 · Sep 26, 2017
commit 6dcc881dc50fc5616f555b3d399bd900521fbad5
diff --git a/lib/mpl_toolkits/axisartist/axes_divider.py b/lib/mpl_toolkits/axisartist/axes_divider.py
@@ -1,8 +1,6 @@
 from __future__ import (absolute_import, division, print_function,
                        unicode_literals)

-from matplotlib.externals import six
-
 from mpl_toolkits.axes_grid1.axes_divider import Divider, AxesLocator, SubplotDivider, \
     AxesDivider, locatable_axes_factory, make_axes_locatable


diff --git a/lib/mpl_toolkits/axisartist/axes_grid.py b/lib/mpl_toolkits/axisartist/axes_grid.py
@@ -1,8 +1,6 @@
 from __future__ import (absolute_import, division, print_function,
                        unicode_literals)

-from matplotlib.externals import six
-
 import mpl_toolkits.axes_grid1.axes_grid as axes_grid_orig
 from .axes_divider import LocatableAxes


diff --git a/lib/mpl_toolkits/axisartist/axes_rgb.py b/lib/mpl_toolkits/axisartist/axes_rgb.py
@@ -1,8 +1,6 @@
 from __future__ import (absolute_import, division, print_function,
                        unicode_literals)

-from matplotlib.externals import six
-
 #from mpl_toolkits.axes_grid1.axes_rgb import *
 from mpl_toolkits.axes_grid1.axes_rgb import make_rgb_axes, imshow_rgb, RGBAxesBase


diff --git a/lib/mpl_toolkits/axisartist/parasite_axes.py b/lib/mpl_toolkits/axisartist/parasite_axes.py
@@ -1,8 +1,6 @@
 from __future__ import (absolute_import, division, print_function,
                        unicode_literals)

-from matplotlib.externals import six
-
 from mpl_toolkits.axes_grid1.parasite_axes import \
     subplot_class_factory, \
     parasite_axes_class_factory, parasite_axes_auxtrans_class_factory, \