Fully-fledged support for mocked objects

Feature

I'd like to propose that fully-fledged support for mocked objects be added so that type checks work with spec'd mocks.

import mock
import typing


class Foo:
    def bar(self, a: int) -> str:
        return str(a)


# I want to pass a spec'd mock here (mock.create_autospec(spec=Foo)).
def test_something(foo: Foo):
    # I'd like to find a way for this to be accepted by mypy.
    # error: "Callable[[int], str]" has no attribute "return_value"
    foo.bar.return_value = "42"


# This can be resolved arduously by writing a custom protocol for each mock.
class BarMockProtocol(typing.Protocol):
    return_value: str

    def __call__(self, a: int) -> str:
        pass

    # def assert_called():
    #     ...
    # And so on...


class FooMockProtocol(typing.Protocol):
    bar: BarMockProtocol


def test_something_else(foo: FooMockProtocol):
    # Error as expected.
    # error: Incompatible types in assignment (expression has type "int", variable has type "str")
    foo.bar.return_value = 42

    # No error as expected.
    foo.bar.return_value = "42"


# This is what I'd suggest
def test_something_else_yet(foo: mock.Mock[Foo]):
    # Error
    foo.bar.return_value = 42
    # No error
    foo.bar.return_value = "42"


# Usage sample
foo_mock = mock.create_autospec(spec=Foo)
test_something_else_yet(foo_mock)

Pitch

Currently, it's difficult to fully utilise mypy with mocks. I can see the following options.

1. Use typing.Any to disable type checks.
2. Use the type of the mocked class and live/deal with errors when using mock-specific attributes and methods (e.g. return_value, assert_*).
3. Write a custom protocol for every mocked class as demonstrated above.

In other words, it's a choice between boilerplate and limitations. Please do let me know if I missed anything.

No implementation files or tests are named. Start by reproducing the issue's Foo, create_autospec, and Mock examples in mypy, then inspect how mocked objects and callable attributes are typed. Done means spec'd mocks support type-checked return_value assignments without custom protocols while preserving errors for incompatible values.