- some changes to the identity map regarding

  - some changes to the identity map regarding
    rare weakref callbacks during iterations.
    The mutex has been removed as it apparently
    can cause a reentrant (i.e. in one thread) deadlock,
    perhaps when gc collects objects at the point of
    iteration in order to gain more memory.  It is hoped
    that "dictionary changed during iteration" will
    be exceedingly rare as iteration methods internally
    acquire the full list of objects in a single values()
    call. [ticket:2087]