bpo-30463: Add an empty __slots__ to abc.ABC.

diff --git a/Lib/abc.py b/Lib/abc.py
index 43a34a0..d13a0de 100644
--- a/Lib/abc.py
+++ b/Lib/abc.py
@@ -235,7 +235,7 @@
     """Helper class that provides a standard way to create an ABC using
     inheritance.
     """
-    pass
+    __slots__ = ()
 
 
 def get_cache_token():