Original patch by Julien Courteau and Pierre Paul Lefebvre.

Issue #16602: When a weakref's target was part of a long deallocation chain, the object could remain reachable through its weakref even though its refcount had dropped to zero.

Thanks to Eugene Toder for diagnosing and reporting the issue.

Patch by Steven Kryskalla.

Thanks to Sean Munkel from docs@

This is necessary for ZFS systems, which don't support UF_IMMUTABLE.
--
Kubilay Kocak (koobs) asked me on IRC to backport this fix to Python 2.7: done!

Patch by Serhiy Storchaka.

Patch by Bruno Dupuis.

The corresponding 3.x commits are 69d5d834c5cb, 40fd26a4206b, and
74d8a2a20c52.

when read several extension modules from the same binary object file.

Patch by Jonathan Kotta.

Patch by Serhiy Storchaka.

initialization. If the interpreter is shut down and reinitialized (embedded
CPython), the old module dictionary was not dec-refed during the next import of
the time extension module.

Contributed by Torsten Marek of Google.