`skip_nonitem_fields=True` doesn't work when the page object is an attrs class

[BurnzZ]

Currently, this works fine:

import attrs
from web_poet import HttpResponse, Returns, ItemPage, field

@attrs.define
class BigItem:
    x: int
    y: int

class BigPage(ItemPage[BigItem]):
    @field
    def x(self):
        return 1

    @field
    def y(self):
        return 2

@attrs.define
class SmallItem:
    x: int

class SmallXPage(BigPage, Returns[SmallItem], skip_nonitem_fields=True):
    pass

page = SmallXPage()
item = await page.to_item()

print(page._skip_nonitem_fields)  # True
print(item)  # SmallItem(x=1)

However, if we define an attrs class to have some page dependencies, it doesn't work:

from web_poet import PageParams

@attrs.define
class SmallPage(BigPage, Returns[SmallItem], skip_nonitem_fields=True):
    params: PageParams

page = SmallPage(params=PageParams())
print(page._skip_nonitem_fields)  # False
item = await page.to_item()  # TypeError: __init__() got an unexpected keyword argument 'y'

From the examples above, this stems from page._skip_nonitem_fields being set to False when the page object is defined as an attrs class.

[BurnzZ]

Related to python-attrs/attrs#971, it all works when turning off slots:

@attrs.define(slots=False)
class SmallPage(BigPage, Returns[SmallItem], skip_nonitem_fields=True):
    params: PageParams