Deprecate unused "frac" key in annotate() arrowprops.

This key has been unused for ~8y (197fc9f).

Author: anntzer

doc/api/next_api_changes/deprecations/26780-AL.rst

@@ -0,0 +1,3 @@
+Support for passing the "frac" key in ``annotate(..., arrowprops={"frac": ...})``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+... has been removed.  This key has had no effect since Matplotlib 1.5.

lib/matplotlib/text.py

@@ -1838,9 +1838,12 @@ def transform(renderer) -> Transform
                 self._arrow_relpos = arrowprops.pop("relpos", (0.5, 0.5))
             else:
                 # modified YAArrow API to be used with FancyArrowPatch
-                for key in [
-                        'width', 'headwidth', 'headlength', 'shrink', 'frac']:
+                for key in ['width', 'headwidth', 'headlength', 'shrink']:
                     arrowprops.pop(key, None)
+                if 'frac' in arrowprops:
+                    _api.warn_deprecated(
+                        "3.8", name="the (unused) 'frac' key in 'arrowprops'")
+                    arrowprops.pop("frac")
             self.arrow_patch = FancyArrowPatch((0, 0), (1, 1), **arrowprops)
         else:
             self.arrow_patch = None
@@ -1933,10 +1936,6 @@ def update_positions(self, renderer):
             shrink = arrowprops.get('shrink', 0.0)
             width = arrowprops.get('width', 4)
             headwidth = arrowprops.get('headwidth', 12)
-            if 'frac' in arrowprops:
-                _api.warn_external(
-                    "'frac' option in 'arrowprops' is no longer supported;"
-                    " use 'headlength' to set the head length in points.")
             headlength = arrowprops.get('headlength', 12)
 
             # NB: ms is in pts