MNT: Make optional parameters to Axes keyword only

This follows the deprecation period.

Author: greglucas

doc/api/next_api_changes/removals/23XXX-GL.rst

@@ -16,3 +16,8 @@ The **minimum_descent** has been removed from ``TextArea``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The minimum_descent is now effectively always True.
+
+Extra parameters to Axes constructor
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Parameters of the Axes constructor other than *fig* and *rect* are now keyword only.

lib/matplotlib/axes/_base.py

@@ -562,8 +562,8 @@ def __str__(self):
         return "{0}({1[0]:g},{1[1]:g};{1[2]:g}x{1[3]:g})".format(
             type(self).__name__, self._position.bounds)
 
-    @_api.make_keyword_only("3.4", "facecolor")
     def __init__(self, fig, rect,
+                 *,
                  facecolor=None,  # defaults to rc axes.facecolor
                  frameon=True,
                  sharex=None,  # use Axes instance's xaxis info