Missing downwards compatibility and bug with custom root types

[camold]

from typing import Generic
from typing import TypeVar
from typing import List
from pydantic.generics import GenericModel
from pydantic import BaseModel

T = TypeVar("T")
class BaseList(GenericModel, Generic[T]):
     __root__: List[T]

class Test(BaseModel):
     mylist: BaseList[int]

Test(mylist=[1,2,3,4])
# Test(mylist=BaseList[int](__root__=[1, 2, 3, 4]))

Test(mylist=BaseList[int](__root__=[1,2,3,4]))
# ---------------------------------------------------------------------------
# ValidationError                           Traceback (most recent call last)
# <ipython-input-10-c373af038c5b> in <module>
# ----> 1 Test(mylist=BaseList[int](__root__=[1,2,3,4]))
# 
# /usr/local/lib/python3.8/dist-packages/pydantic/main.cpython-38-x86_64-linux-gnu.so in pydantic.main.BaseModel.__init__()
# 
# ValidationError: 1 validation error for Test
# mylist -> __root__
#   value is not a valid list (type=type_error.list)

Certainly no error with pydantic 1.6.1 (I tested this locally).
Probably also no errors with versions <1.8 because we have automatic tests in place with the newest package versions and these tests only fired yesterday.

On a first glance the documentation does not seem to have changed - so I assume also the behaviour should not have changed with the 1.8 release. See https://pydantic-docs.helpmanual.io/usage/models/#generic-models and
https://pydantic-docs.helpmanual.io/usage/models/#custom-root-types

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.8
            pydantic compiled: True
                 install path: /usr/local/lib/python3.8/dist-packages/pydantic
               python version: 3.8.0 (default, Oct 28 2019, 16:14:01)  [GCC 8.3.0]
                     platform: Linux-5.4.0-62-generic-x86_64-with-glibc2.27
     optional deps. installed: ['devtools', 'email-validator', 'typing-extensions']
...