Issue #12400: test_cprofile now restores correctly the previous sys.stderr

Copy sys.stderr before replacing it, instead of using sys.__stderr__

Lib/test/test_cprofile.py

@@ -18,6 +18,8 @@ class CProfileTest(ProfileTest):
     def test_bad_counter_during_dealloc(self):
         import _lsprof
         # Must use a file as StringIO doesn't trigger the bug.
+        orig_stderr = sys.stderr
+        try:
             with open(TESTFN, 'w') as file:
                 sys.stderr = file
                 try:
@@ -26,7 +28,8 @@ class CProfileTest(ProfileTest):
                     obj = _lsprof.Profiler(1)
                     obj.disable()
                 finally:
-                sys.stderr = sys.__stderr__
+                    sys.stderr = orig_stderr
+        finally:
             unlink(TESTFN)