Deprecate font_manager.is_opentype_cff_font

According to the docs, it was used for PostScript and PDF which "cannot
subset those fonts". However, that is no longer true, and there are no
users of this function.

Author: QuLogic

doc/api/next_api_changes/deprecations/30329-ES.rst

@@ -0,0 +1,4 @@
+``font_manager.is_opentype_cff_font`` is deprecated
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There is no replacement.

lib/matplotlib/font_manager.py

@@ -1539,7 +1539,7 @@ def _findfont_cached(self, prop, fontext, directory, fallback_to_default,
         return _cached_realpath(result)
 
 
-@lru_cache
+@_api.deprecated("3.11")
 def is_opentype_cff_font(filename):
     """
     Return whether the given font is a Postscript Compact Font Format Font

lib/matplotlib/tests/test_font_manager.py

@@ -67,12 +67,14 @@ def test_json_serialization(tmp_path):
 def test_otf():
     fname = '/usr/share/fonts/opentype/freefont/FreeMono.otf'
     if Path(fname).exists():
-        assert is_opentype_cff_font(fname)
+        with pytest.warns(mpl.MatplotlibDeprecationWarning):
+            assert is_opentype_cff_font(fname)
     for f in fontManager.ttflist:
         if 'otf' in f.fname:
             with open(f.fname, 'rb') as fd:
                 res = fd.read(4) == b'OTTO'
-            assert res == is_opentype_cff_font(f.fname)
+            with pytest.warns(mpl.MatplotlibDeprecationWarning):
+                assert res == is_opentype_cff_font(f.fname)
 
 
 @pytest.mark.skipif(sys.platform == "win32" or not has_fclist,