Simplify _api.warn_external on Python 3.12+

Python 3.12 added the `skip_file_prefixes` argument, which essentially
does what this helper function does for us.

Technically, I think the old implementation would set the stack level to
the tests, if called in one, but this one doesn't. However, that
shouldn't be a problem, as either 1) warnings are errors, or 2), we
catch the warning with `pytest.warns` and don't see the stack level.

Author: QuLogic

lib/matplotlib/_api/__init__.py

@@ -366,16 +366,22 @@ def warn_external(message, category=None):
     warnings.warn`` (or ``functools.partial(warnings.warn, stacklevel=2)``,
     etc.).
     """
-    frame = sys._getframe()
-    for stacklevel in itertools.count(1):
-        if frame is None:
-            # when called in embedded context may hit frame is None
-            break
-        if not re.match(r"\A(matplotlib|mpl_toolkits)(\Z|\.(?!tests\.))",
-                        # Work around sphinx-gallery not setting __name__.
-                        frame.f_globals.get("__name__", "")):
-            break
-        frame = frame.f_back
-    # preemptively break reference cycle between locals and the frame
-    del frame
-    warnings.warn(message, category, stacklevel)
+    kwargs = {}
+    if sys.version_info[:2] >= (3, 12):
+        kwargs['skip_file_prefixes'] = ('matplotlib', 'mpl_toolkits')
+    else:
+        frame = sys._getframe()
+        for stacklevel in itertools.count(1):
+            if frame is None:
+                # when called in embedded context may hit frame is None
+                kwargs['stacklevel'] = stacklevel
+                break
+            if not re.match(r"\A(matplotlib|mpl_toolkits)(\Z|\.(?!tests\.))",
+                            # Work around sphinx-gallery not setting __name__.
+                            frame.f_globals.get("__name__", "")):
+                kwargs['stacklevel'] = stacklevel
+                break
+            frame = frame.f_back
+        # preemptively break reference cycle between locals and the frame
+        del frame
+    warnings.warn(message, category, **kwargs)