kw_only is only supported in Python >=3.10

[antoinevg]

Just noticed that our current min Python version is 3.9.x which does not support kw_only. 😔

This isn't great because I can't think of another clean way to do:

facedancer/facedancer/magic.py

Line 23 in afeb3b7

return dataclass(new_cls, kw_only=True)

and

facedancer/facedancer/magic.py

Line 31 in afeb3b7

cls.__init__.__kwdefaults__[name] = value

Anyone else have ideas, short of bumping min Python version to 3.10?