Slightly tighten the _get_layout_cache_key API.

_get_layout_cache_key is only ever called from _get_layout, which always
passes a renderer arg which is never None (_get_layout calls
`renderer.get_text_width_height_descent immediately after).  So we can
make the `renderer` arg required and not check for None.

Also, get_unitless_position() can be directly used instead of unpacking
it.

Author: anntzer

lib/matplotlib/text.py

@@ -272,15 +272,14 @@ def update_from(self, other):
         self._linespacing = other._linespacing
         self.stale = True
 
-    def _get_layout_cache_key(self, renderer=None):
+    def _get_layout_cache_key(self, renderer):
         """
         Return a hashable tuple of properties that lets `_get_layout` know
         whether a previously computed layout can be reused.
         """
-        x, y = self.get_unitless_position()
-        renderer = renderer or self._renderer
         return (
-            x, y, self.get_text(), hash(self._fontproperties),
+            self.get_unitless_position(), self.get_text(),
+            hash(self._fontproperties),
             self._verticalalignment, self._horizontalalignment,
             self._linespacing,
             self._rotation, self._rotation_mode, self._transform_rotates_text,