fix bug where black.assert_equivalent no longer raises an AssertionError

[spyoungtech]

psf/black#4270 changed black.assert_equivalent to raise an ASTSafetyError exception (a custom exception inheriting from Exception) so catching AssertionError no longer works when developing with recent versions of black

This change makes the code compatible with older and newer versions of black.

[coveralls]

coverage: 77.477%. remained the same
when pulling 1753204 on fix-pre-commit-hook
into de90e94 on main.