* Remove unused SSLProtocol._closing attribute
* test_sslproto: skip test if ssl module is missing
* Python issue #23208: Don't use the traceback of the current handle if we
already know the traceback of the source. The handle may be more revelant,
but having 3 tracebacks (handle, source, exception) becomes more difficult to
read. The handle may be preferred later but it requires more work to make
this choice.
* Use test_utils.run_briefly() to execute pending calls to really close
transports
* sslproto: mock also _SSLPipe.shutdown(), it's need to close the transport
* pipe test: the test doesn't close explicitly the PipeHandle, so ignore
the warning instead
* test_popen: use the context manager ("with p:") to explicitly close pipes
Victor Stinner