Issue #12400: fix test_faulthandler if regrtest captures sys.stderr

faulthandler.enable() requires that sys.stderr has a fileno() method.

Lib/test/test_faulthandler.py

 from contextlib import contextmanager
 import datetime
 import faulthandler
+import os
 import re
 import signal
 import subprocess
@@ -230,6 +231,15 @@ faulthandler._read_null()
         self.assertNotEqual(exitcode, 0)
 
     def test_is_enabled(self):
+        null_stderr = None
+        orig_stderr = sys.stderr
+        try:
+            # regrtest may replace sys.stderr by io.StringIO object, but
+            # faulthandler.enable() requires that sys.stderr has a fileno()
+            # method
+            null_stderr = open(os.devnull, 'w')
+            sys.stderr = null_stderr
+
             was_enabled = faulthandler.is_enabled()
             try:
                 faulthandler.enable()
@@ -241,6 +251,10 @@ faulthandler._read_null()
                     faulthandler.enable()
                 else:
                     faulthandler.disable()
+        finally:
+            sys.stderr = orig_stderr
+            if null_stderr is not None:
+                null_stderr.close()
 
     def check_dump_traceback(self, filename):
         """