MNT: delete references to frame objects

This is to help prevent reference cycles between the frames and the locals in
that frame.

While garbage collection should in most cases take care of this eventually, we
can help the interpreter out and break the cycle before we return.

Author: tacaswell

lib/matplotlib/_api/__init__.py

@@ -385,4 +385,6 @@ def warn_external(message, category=None):
                         frame.f_globals.get("__name__", "")):
             break
         frame = frame.f_back
+    # premetively break reference cycle between locals and the frame
+    del frame
     warnings.warn(message, category, stacklevel)

lib/matplotlib/cbook.py

@@ -67,6 +67,8 @@ def _get_running_interactive_framework():
                 if frame.f_code in codes:
                     return "tk"
                 frame = frame.f_back
+        # premetively break reference cycle between locals and the frame
+        del frame
     macosx = sys.modules.get("matplotlib.backends._macosx")
     if macosx and macosx.event_loop_is_running():
         return "macosx"