Add mypy plugin

[giannitedesco]

This is a first cut, just to prove the concept.

I can run mypy --show-traceback --strict on the following script and it works:

from dataclassy import dataclass

@dataclass
class Foo:
    a: int
    b: str = 'default'

    def method(self) -> str:
        return f'{self.b}/{self.a}'

class Bar(Foo):
    c: float

w = Foo(1, 'abc')
x = Foo(2)
y = Bar(3, 4.5)
z = Bar(6, 7.8, 'now')

I think this still needs to support various options like init=False, generation of __eq__ and __lt__ methods before we should merge it and I'll work on that next.

Are there any other features I need to support, I wonder? Probably I should support make_dataclass() in some way? Perhaps that can only work when the inputs are all constant expressions - I will have a look to see how namedtuple() is handled, since that is a similar kind of thing. We can probably live without that for now?

[biqqles]

Nice progress! The semantics of make_dataclass should be identical to dataclasses' version, except for the flexible inheritance. It's used so rarely that for now you could simply fall back to the stdlib version imo.

[giannitedesco]

I just had a look and it seems that the existing dataclasses plugin doesn't even support that. Furthermore, namedtuple() has specific support in mypy semantic analyzer (not a plugin). So i'm quite happy leaving that off the list for now.

My plan is to add __eq__, and __lt__ and understand relevant decorator args this weekend and try use it a bit in anger and then ready-for-review the PR.

[giannitedesco]

Been a bit busy with other projects, but things are slowly progressing. All decorator arguments are now parsed, checking works properly in the case of @dataclassy(init=False). I will add logic for __eq__, __lt__, __repro__, and __iter__, methods next. That's all pretty straightforward.

I think there also needs to be some logic for when __hash__ method gets created and for what to do when kwargs=True.

[giannitedesco]

Okay, so nothing needs to be done for __eq__, __lt__, __repr__.

Dealing with __iter__ is either impossible or complicated, you will get ~type Any for any fields~ a "type is not Iterable" error when unpacking dataclassy type that was given decorator param iter=True. I will need some help from mypy people to solve that, I think.

I haven't looked at __hash__ properly but I am suspecting the same sort of issues there.

For these reasons, I will make this ready for review as a pretty usable first cut. And I will try and figure out the rest later!

[biqqles]

Very, very nice work, thank you!