Removed BaseCommand.stdin introduced in 116d39842d.

This option is not actually very useful in the general case
because it doesn't override sys.stdin.

It's still marginally useful for testing some features of
the createsuperuser command so it was moved there.
This commit also makes the detection of a TTY in createsuperuser
a bit more robust, after a suggestion of appolo13.

tests/user_commands/tests.py

@@ -61,32 +61,6 @@ class CommandTests(SimpleTestCase):
             management.call_command('leave_locale_alone_true', stdout=out)
             self.assertEqual(out.getvalue(), "pl\n")
 
-    def test_passing_stdin(self):
-        """
-        You can pass a stdin object to a command's options and it should be
-        available on self.stdin.
-        """
-        class CustomCommand(BaseCommand):
-            def handle(self, *args, **kwargs):
-                pass
-
-        sentinel = object()
-        command = CustomCommand()
-        command.execute(stdin=sentinel, stdout=StringIO())
-        self.assertIs(command.stdin, sentinel)
-
-    def test_passing_stdin_default(self):
-        """
-        If stdin is not passed as an option, the default should be sys.stdin.
-        """
-        class CustomCommand(BaseCommand):
-            def handle(self, *args, **kwargs):
-                return 'OK'
-
-        command = CustomCommand()
-        command.execute(stdout=StringIO())
-        self.assertIs(command.stdin, sys.stdin)
-
 
 class UtilsTests(SimpleTestCase):