Deprecate inset_locator.InsetPosition.

There's Axes.inset_axes which is an exact replacement without the need
of actually understanding what an axes locator is, and which is also
more general as one can also set a custom transform on inset_axes
instead of being limited to the transAxes of a given parent axes.

Also the method had never been used in the library or examples, and a
test was only ever added in 6dd30a2 (2022).

Author: anntzer

doc/api/next_api_changes/deprecations/26295-AL.rst

@@ -0,0 +1,3 @@
+``inset_location.InsetPosition`` is deprecated
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Use `~.Axes.inset_axes` instead.

lib/mpl_toolkits/axes_grid1/inset_locator.py

@@ -13,6 +13,7 @@
 from .parasite_axes import HostAxes
 
 
+@_api.deprecated("3.8", alternative="Axes.inset_axes")
 class InsetPosition:
     @_docstring.dedent_interpd
     def __init__(self, parent, lbwh):

lib/mpl_toolkits/axes_grid1/tests/test_axes_grid1.py

@@ -691,7 +691,8 @@ def test_rgb_axes():
 def test_insetposition():
     fig, ax = plt.subplots(figsize=(2, 2))
     ax_ins = plt.axes([0, 0, 1, 1])
-    ip = InsetPosition(ax, [0.2, 0.25, 0.5, 0.4])
+    with pytest.warns(mpl.MatplotlibDeprecationWarning):
+        ip = InsetPosition(ax, [0.2, 0.25, 0.5, 0.4])
     ax_ins.set_axes_locator(ip)