Example script not valid for Python <3.9

[Alex Fornuto]

Not all (perhaps even not many) operating systems ship Python 3.9+ in their repos.

❯ SERVICE_ACCOUNT="REDACTED" python new-route.py
Traceback (most recent call last):
  File "new-route-custom.py", line 4, in <module>
    from pomerium.client import Client
  File "/home/alex/.local/lib/python3.8/site-packages/pomerium/client.py", line 35, in <module>
    def _dial(target: str, auth_token: str, root_certificates: bytes = None, private_key: bytes = None, certificate_chain: bytes = None, options: list[tuple[str, str]] = None):
TypeError: 'type' object is not subscriptable
❯ python --version
Python 3.8.10

Relevant line: TypeError: 'type' object is not subscriptable