Note Python 3.11 dependency in README

[barborico]

Access requires at least Python 3.11 to use StrEnum:

(venv) ➜  access git:(main) flask db upgrade
Error: While importing 'api.app', an ImportError was raised:

Traceback (most recent call last):
  File "/home/discord/workspaces/access/venv/lib/python3.10/site-packages/flask/cli.py", line 245, in locate_app
    __import__(module_name)
  File "/home/discord/workspaces/access/api/app.py", line 19, in <module>
    from api.authentication import AuthenticationHelpers, CloudflareAuthenticationHelpers
  File "/home/discord/workspaces/access/api/authentication.py", line 14, in <module>
    from api.models import OktaUser
  File "/home/discord/workspaces/access/api/models/__init__.py", line 1, in <module>
    from api.models.core_models import (
  File "/home/discord/workspaces/access/api/models/core_models.py", line 2, in <module>
    from enum import StrEnum
ImportError: cannot import name 'StrEnum' from 'enum' (/usr/lib/python3.10/enum.py)