bpo-38912: regrtest logs unraisable exception into sys.__stderr__ (GH-21718)

regrtest_unraisable_hook() temporarily replaces sys.stderr with
sys.__stderr__ to help to display errors when a test captures stderr.
diff --git a/Lib/test/libregrtest/utils.py b/Lib/test/libregrtest/utils.py
index 0368694..71f538f 100644
--- a/Lib/test/libregrtest/utils.py
+++ b/Lib/test/libregrtest/utils.py
@@ -72,7 +72,12 @@
     global orig_unraisablehook
     support.environment_altered = True
     print_warning("Unraisable exception")
-    orig_unraisablehook(unraisable)
+    old_stderr = sys.stderr
+    try:
+        sys.stderr = sys.__stderr__
+        orig_unraisablehook(unraisable)
+    finally:
+        sys.stderr = old_stderr
 
 
 def setup_unraisable_hook():