Python 3.11 compatibility: dataclass default factory.

[mattf1n]

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Issue #29 describes the problem. To fix it, we use a field with a default_factory.

[srush]

This looks great. Was planning on updating to Python 3.10 minimum this summer since there are some nice type system improvements.

Is there a reason it is a draft?

[mattf1n]

I put it as a draft because I didn't do any testing, just made the minimal edit so that I could import again and run my code. I will promote it now.