bpo-13601: always use line-buffering for sys.stderr

Add versionchanged note to docs.
Add tests.
python · jendrikseipp · jendrikseipp · jendrikseipp · Dec 17, 2019 · the-knights-who-say-ni
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
diff --git a/Lib/test/test_cmd_line.py b/Lib/test/test_cmd_line.py
+            code = f'import os, sys; {buf}.write({txt1!r}); {buf}.write({txt2!r}); os._exit(0)'
+            rc, out, err = assert_python_ok('-c', code)
+            self.assertEqual(out, out_ok)
+            self.assertEqual(err, err_ok)