MNT: Prevent users from erroneously using legend label API on Axis

Closes matplotlib#27971.

For a complete explanation see matplotlib#27971 (comment)

doc/api/next_api_changes/deprecations/28584-TH.rst

@@ -0,0 +1,6 @@
+``Axis.get_label``
+~~~~~~~~~~~~~~~~~~
+`.Artist.get_label` is reserved for legend labels. `.Axis.get_label` had overwritten
+this, providing the axis label `.Text` instance. `.Axis.get_label` will raise a
+RuntimeError in the future. If you want the axis label `.Text` instance, use the
+property `.Axis.label` instead.

lib/matplotlib/axis.py

@@ -1442,6 +1442,15 @@ def get_gridlines(self):
         return cbook.silent_list('Line2D gridline',
                                  [tick.gridline for tick in ticks])
 
+    def set_label(self, s):
+        """Not supported. Raises RuntimeError."""
+        raise RuntimeError(
+            "A legend label cannot be assigned to an Axis. Did you mean to "
+            "set the axis label via set_label_text()?")
+
+    @_api.deprecated("3.10", message="Use the property 'Axis.label' instead")
+    # Note: After the deprecation, this should raise a RuntimeError; see
+    # https://github.com/matplotlib/matplotlib/issues/27971#issuecomment-2016955731
     def get_label(self):
         """Return the axis label as a Text instance."""
         return self.label