pmb.config.sudo: Use type union compatible with Python 3.7

The Type | OtherType syntax for writing unions was introduced in Python
3.10. We want to support Python 3.7, so use an Optional type hint
instead which declares the same thing and is supported by 3.7.

Fixes: d31313f7 ("Don't use 'sudo' when running as root")
Reviewed-by: Luca Weiss <luca@z3ntu.xyz>
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20230605063142.6843-1-newbyte@postmarketos.org%3E

pmb/config/sudo.py

@@ -3,10 +3,11 @@
 import os
 import shutil
 from functools import lru_cache
+from typing import Optional
 
 
 @lru_cache()
-def which_sudo() -> str | None:
+def which_sudo() -> Optional[str]:
     """Returns a command required to run commands as root, if any.
 
     Find whether sudo or doas is installed for commands that require root.