__slots__ are ignored

Hey there! First, thanks for this great library, I am really enjoying using it!

Sometimes I like to use __slots__ with dataclasses to prevent me from accidentally setting a value to a (typo-ed) new attribute instead of to an existing attribute.

However, if I use the DataClassDictMixin from mashumaro, it seems like __slots__ are ignored. I.e. If I set a value to a non-existing attribute, I do not get AttributeError, but a new attribute is created and value is set to it, as if __slots__ would not exist.

Here’s a short pytest case to showcase the problem:

import pytest
from dataclasses import dataclass
from mashumaro import DataClassDictMixin


@dataclass
class Foo:
    __slots__ = ["number"]
    number: int


@dataclass
class Bar(DataClassDictMixin):
    __slots__ = ["number"]
    number: int


def test_slots():

    foo = Foo(1)
    with pytest.raises(AttributeError) as err:
        foo.new_attribute = 2
    assert str(err.value) == "'Foo' object has no attribute 'new_attribute'"

    bar = Bar(1)
    bar.new_attribute = 2
    # -> should also fail with "'Bar' object has no attribute 'new_attribute'",
    # but it doesn't

[Fatal1ty]

Fixed in 2.2.

[zupo]

https://github.com/Fatal1ty/mashumaro/commit/543356e4add2e215f90d36e011efc489e0d2e936 is definitely needed.

I was not using the correct branch when I first tested (I thought I did) and now I re-checked and works as expected with the slots branch.

I closed the issue by mistake. Getting late, I should go to bed.