bpo-45410: regrtest -W leaves stdout/err FD unchanged (GH-28915)

support.print_warning() now stores the original value of
sys.__stderr__ and uses it to log warnings. libregrtest uses the same
stream to log unraisable exceptions and uncaught threading
exceptions.

Partially revert commit dbe213de7e:
libregrtest no longer replaces sys.__stdout__, sys.__stderr__, and
stdout and stderr file descriptors.

Remove also a few unused imports in libregrtest.

Lib/test/libregrtest/cmdline.py

@@ -1,7 +1,6 @@
 import argparse
 import os
 import sys
-from test import support
 from test.support import os_helper