Issue 18772:  Restore set dummy object back to unicode and restore the identity checks in lookkey().

The Gdb prettyprint plugin depended on the dummy object being displayable.
Other solutions besides a unicode object are possible.  For now, get it
back up and running.

The identity checks in lookkey() need to be there to prevent the dummy
object from leaking through Py_RichCompareBool() into user code in the
rare circumstance where the dummy's hash value exactly matches the hash
value of the actual key being looked up.
Remove a redundant hash table probe (this was artifact from an earlier draft of the patch).
Hoist the global dummy lookup out of the inner loop for set_merge().
Issue #18747: Re-seed OpenSSL's pseudo-random number generator after fork.
A pthread_atfork() child handler is used to seeded the PRNG with pid, time
and some stack data.
Issue #18747: Re-seed OpenSSL's pseudo-random number generator after fork.
A pthread_atfork() child handler is used to seeded the PRNG with pid, time
and some stack data.
Issue #17119: Fixed integer overflows when processing large strings and tuples
in the tkinter module.
Issue #17119: Fixed integer overflows when processing large strings and tuples
in the tkinter module.
Issue #18762: Print debug info on failure to create new forkserver process.

Also modify test code to hopefully avoid deadlock on failure.
Merge heads