Typing Error on Python 3.9

[abhishek9sharma]

https://github.com/guardrails-ai/guardrails_pii/blob/main/validator/main.py#L77

Above line gives below error on python3.9

  File "/home/abc/ENTER/envs/gr_env/lib/python3.9/site-packages/guardrails_grhub_guardrails_pii/main.py", line 77, in GuardrailsPII
    entities: str | List[str],
TypeError: unsupported operand type(s) for |: 'type' and '_GenericAlias'

I think can be updated to below

entities: Union[str, List[str]],

Let me know if it makes sense and I will raise a PR @CalebCourier @zsimjee