compare singletons by identity not equality (closes #16712)

Patch from Serhiy Storchaka.
diff --git a/Tools/gdb/libpython.py b/Tools/gdb/libpython.py
index 2cdc3da..368a7d5 100644
--- a/Tools/gdb/libpython.py
+++ b/Tools/gdb/libpython.py
@@ -1332,7 +1332,7 @@
   /usr/lib/debug/usr/lib/libpython2.6.so.1.0.debug-gdb.py
 """
 def register (obj):
-    if obj == None:
+    if obj is None:
         obj = gdb
 
     # Wire up the pretty-printer