kifparser fails to start

[leungmanhin]

Hi, I'm getting this error when starting the kirparser:

$ kifparser
Traceback (most recent call last):
  File ".dev-venv/bin/kifparser", line 27, in <module>
    from kifparser import KIFParser, KIFGrammar
  File ".dev-venv/lib/python3.8/site-packages/kifparser/__init__.py", line 21, in <module>
    from .kifparser import KIFParser, KIFAgenda
  File ".dev-venv/lib/python3.8/site-packages/kifparser/kifparser.py", line 25, in <module>
    from aiire import Agenda
  File ".dev-venv/lib/python3.8/site-packages/aiire/__init__.py", line 8, in <module>
    from .grammar import Grammar
  File ".dev-venv/lib/python3.8/site-packages/aiire/grammar.py", line 14, in <module>
    class Grammar(object):
  File ".dev-venv/lib/python3.8/site-packages/aiire/grammar.py", line 154, in Grammar
    class Constituent(object):
  File ".dev-venv/lib/python3.8/site-packages/aiire/grammar.py", line 216, in Constituent
    children: tuple['Grammar.Constituent'] = None
TypeError: 'type' object is not subscriptable