Use BlackRunner and .stdout in command line test

So the next commit won't break this test. This is in its own commit so
we can just revert the depreciation commit when dropping Python 2
support completely.

tests/test_black.py

@@ -1400,14 +1400,14 @@ def test_docstring_reformat_for_py27(self) -> None:
         )
         expected = 'def foo():\n    """Testing\n    Testing"""\n    print "Foo"\n'
 
-        result = CliRunner().invoke(
+        result = BlackRunner().invoke(
             black.main,
             ["-", "-q", "--target-version=py27"],
             input=BytesIO(source),
         )
 
         self.assertEqual(result.exit_code, 0)
-        actual = result.output
+        actual = result.stdout
         self.assertFormatEqual(actual, expected)
 
     @staticmethod