hook: lower inflated use of sys.__stdout__ and __stderr__

They were introduced at 9f76df0edb7d, where sys.stdout could be replaced by
sys.stderr. After that, we've changed the way of stdout redirection by
afccc64eea73, so we no longer need to reference the original __stdout__ and
__stderr__ objects.

Let's move away from using __std*__ objects so we can simply wrap sys.std*
objects for Python 3 porting.

mercurial/hook.py

@@ -209,11 +209,11 b' def runhooks(ui, repo, name, hooks, thro'
         for hname, cmd in hooks:
             if oldstdout == -1 and _redirect:
                 try:
-                    stdoutno = sys.__stdout__.fileno()
-                    stderrno = sys.__stderr__.fileno()
+                    stdoutno = sys.stdout.fileno()
+                    stderrno = sys.stderr.fileno()
                     # temporarily redirect stdout to stderr, if possible
                     if stdoutno >= 0 and stderrno >= 0:
-                        sys.__stdout__.flush()
+                        sys.stdout.flush()
                         oldstdout = os.dup(stdoutno)
                         os.dup2(stderrno, stdoutno)
                 except (OSError, AttributeError):
@@ -258,7 +258,7 b' def runhooks(ui, repo, name, hooks, thro'
             sys.stderr.flush()
     finally:
         if _redirect and oldstdout >= 0:
-            sys.__stdout__.flush()  # write hook output to stderr fd
+            sys.stdout.flush()  # write hook output to stderr fd
             os.dup2(oldstdout, stdoutno)
             os.close(oldstdout)