Fix "Exception ignored" in weakref callback during interpreter shutdown.

Under some circumstances "Exception ignored in: <function
WeakMethod.__new__.<locals>._cb at 0x770cbdb5b9c0>" appears during interpreter shutdown.

Root cause is probably a weakref callback running here from an interpeter shutdown
finalizer pass, and then calling _disconnect either causes the same weakref to be
cleaned up again or calls into some other already-gone resource and triggers the error.

CHANGES.rst

@@ -6,6 +6,8 @@ Unreleased
 -   Drop support for Python 3.8. :pr:`175`
 -   Remove previously deprecated ``__version__``, ``receiver_connected``,
     ``Signal.temporarily_connected_to`` and ``WeakNamespace``. :pr:`172`
+-   Skip weakref signal cleanup if the interpreter is shutting down.
+    :issue:`173`
 
 
 Version 1.8.2

src/blinker/base.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import collections.abc as c
+import sys
 import typing as t
 import weakref
 from collections import defaultdict
@@ -403,7 +404,10 @@ def _make_cleanup_receiver(
         """
 
         def cleanup(ref: weakref.ref[c.Callable[..., t.Any]]) -> None:
-            self._disconnect(receiver_id, ANY_ID)
+            # If the interpreter is shutting down, disconnecting can result in a
+            # weird ignored exception. Don't call it in that case.
+            if not sys.is_finalizing():
+                self._disconnect(receiver_id, ANY_ID)
 
         return cleanup