When subprocess.Popen() stdin= stdout= or stderr= handles are specified
and appear in pass_fds=, don't close the original fds after dup'ing them.

This implementation and unittest primarily came from @izbyshev (see the PR)

See also https://github.com/izbyshev/cpython/commit/b89b52f28490b69142d5c061604b3a3989cec66c

This also removes the old manual p2cread, c2pwrite, and errwrite closing logic
as inheritable flags and _close_open_fds takes care of that properly today without special treatment.

This code is within child_exec() where it is the only thread so there is no
race condition between the dup and _Py_set_inheritable_async_safe call.

* bpo-33217: Raise TypeError for non-Enum lookups in Enums

HMAC's digestmod was deprecated marked for removal, this removes it as planned.

The recursive frame pruning code always undercounted the number of elided frames
by one. That is, in the "[Previous line repeated N more times]" message, N would
always be one too few. Near the recursive pruning cutoff, one frame could be
silently dropped. That situation is demonstrated in the OP of the bug report.

The fix is to start the identical frame counter at 1.

also mention the change and its consequences in What's New

* Make its default value an empty tuple instead of None.
* Make it a keyword-only parameter.

* Make sure that when some of the tests in test_smtplib fail, the allocated threads
and sockets are not leaked. 

* Use support.join_thread() instead of thread.join() to avoid infinite blocks.

Some methods of the SMTP class use mutable default arguments. Specially
`send_message` is affected as it mutates one of the args by appending items
to it, which has side effects on further calls.