Fix Lines type alias definition

[ramiro]

Currently it fails with

Traceback (most recent call last):
  File "~/.pyenv/versions/3.7.9/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "~/.pyenv/versions/3.7.9/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "~/.virtualenvs/tmp-32ba2626ac1e0/lib/python3.7/site-packages/textual/app.py", line 18, in <module>
    from . import events
  File "~/.virtualenvs/tmp-32ba2626ac1e0/lib/python3.7/site-packages/textual/events.py", line 7, in <module>
    from .message import Message
  File "~/.virtualenvs/tmp-32ba2626ac1e0/lib/python3.7/site-packages/textual/message.py", line 7, in <module>
    from ._types import MessageTarget
  File "~/.virtualenvs/tmp-32ba2626ac1e0/lib/python3.7/site-packages/textual/_types.py", line 31, in <module>
    Lines = list[list[Segment]]
TypeError: 'type' object is not subscriptable

[willmcgugan]

Thanks