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/runtest_mp.py

@@ -1,4 +1,3 @@
-import collections
 import faulthandler
 import json
 import os