Typing issue (stubs) for converter with default

When using attr.ibute with both converter and default, wrong expectations is applied on default type (with mypy == 0.670). Mypy dev say it's an issue in attr stubs.

The valid code

import attr
import typing

def to_int(x: typing.Any) -> int:
    return int(x)

@attr.s
class C:
    a: int = attr.ib(converter=to_int, default="1")

results in:

main.py:9: error: Incompatible types in assignment (expression has type "str", variable has type "int")
main.py:9: error: Argument "converter" has incompatible type "Callable[[Any], int]"; expected "Optional[Callable[[Any], str]]"

The result is not right, because the converter is applied on default value as well.

Of course the hotfix is to convert the default value manually.

Reproduce the issue with the provided Python example and mypy 0.670, then inspect the attrs stubs involved in attr.ib typing for converter and default. Done means the example no longer produces the reported assignment and converter errors, with the corrected behavior covered by a regression test.