Settle on "utf8" instead of "utf-8"

tests/test_black.py

@@ -150,7 +150,7 @@ def test_expression_ff(self) -> None:
         tmp_file = Path(black.dump_to_file(source))
         try:
             self.assertTrue(ff(tmp_file, write_back=black.WriteBack.YES))
-            with open(tmp_file, encoding="utf-8") as f:
+            with open(tmp_file, encoding="utf8") as f:
                 actual = f.read()
         finally:
             os.unlink(tmp_file)