pmbootstrap no longer works with python < 3.10

After commit 18fa4e58 ("Ruff: fix typing.Xxx is deprecated, use xxx instead") running pmbootstrap with python3.9 gives:

 $ pmbootstrap --help
Traceback (most recent call last):
  File "/home/grimler/pmbootstrap/pmbootstrap.py", line 7, in <module>
    import pmb
  File "/home/grimler/pmbootstrap/pmb/__init__.py", line 14, in <module>
    from . import config
  File "/home/grimler/pmbootstrap/pmb/config/__init__.py", line 5, in <module>
    from pmb.types import AportGenEntry, PathString
  File "/home/grimler/pmbootstrap/pmb/types.py", line 34, in <module>
    class PmbArgs(Namespace):
  File "/home/grimler/pmbootstrap/pmb/types.py", line 117, in PmbArgs
    package: str | list[str]
TypeError: unsupported operand type(s) for |: 'type' and 'types.GenericAlias'

Should we bump the minimum supported python version, or do we want to try to support python 3.9 as well?

(I think it works with 3.10 if I am reading https://peps.python.org/pep-0604/ correctly, but haven't verified it)