bpo-33263: Fix FD leak in _SelectorSocketTransport (GH-6450)

* bpo-33263 Fix FD leak in _SelectorSocketTransport. (GH-6450)

Under particular circumstances _SelectorSocketTransport can try to add a reader
even the transport is already being closed. This can lead to FD leak and
invalid stated of the following connections. Fixed the SelectorSocketTransport
to add the reader only if the trasport is still active.