Hint at likely cause of ast parsing failure in error message (#2786)

Co-authored-by: Batuhan Taskaya <isidentical@gmail.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>

src/black/__init__.py

@@ -1312,7 +1312,10 @@ def assert_equivalent(src: str, dst: str, *, pass_num: int = 1) -> None:
         src_ast = parse_ast(src)
     except Exception as exc:
         raise AssertionError(
-            f"cannot use --safe with this file; failed to parse source file: {exc}"
+            f"cannot use --safe with this file; failed to parse source file AST: "
+            f"{exc}\n"
+            f"This could be caused by running Black with an older Python version "
+            f"that does not support new syntax used in your source file."
         ) from exc
 
     try: