Prepare for Python 2 depreciation

- 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.

CHANGES.md

@@ -8,6 +8,7 @@
 - Add new `--workers` parameter (#2514)
 - Fixed feature detection for positional-only arguments in lambdas (#2532)
 - Bumped typed-ast version minimum to 1.4.3 for 3.10 compatiblity (#2519)
+- Deprecate Python 2 formatting support (#2523)
 
 ### _Blackd_
 

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