Deprecate Substitution.from_params.

This method has never, ever been used.  It is readily replaced, if truly
needed (I would consider Substitution to be semi-private API
anyways...), by creating an empty Substitution object and assigning to
its params attribute.

Author: anntzer

doc/api/next_api_changes/deprecations.rst

@@ -333,3 +333,8 @@ In :mod:`mpl_toolkits.axes_grid1.axes_rgb`, ``imshow_rgb`` is deprecated (use
 ``ax.imshow(np.dstack([r, g, b]))`` instead); ``RGBAxesBase`` is deprecated
 (use ``RGBAxes`` instead); ``RGBAxes.add_RGB_to_figure`` is deprecated (it was
 an internal helper).
+
+``Substitution.from_params``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This method is deprecated.  If needed, directly assign to the ``params``
+attribute of the Substitution object.

lib/matplotlib/docstring.py

@@ -1,5 +1,7 @@
 import inspect
 
+from matplotlib import cbook
+
 
 class Substitution:
     """
@@ -45,6 +47,7 @@ def update(self, *args, **kwargs):
         self.params.update(*args, **kwargs)
 
     @classmethod
+    @cbook.deprecated("3.3", alternative="assign to the params attribute")
     def from_params(cls, params):
         """
         In the case where the params is a mutable sequence (list or