autodoc can't detect overloaded methods defined in other file

[tk0miya]

Describe the bug
autodoc can't detect overloaded methods defined in other file.

To Reproduce

# example.py
from typing import overload


class Foo:
    @overload
    def __init__(self, x: int) -> None:
        ...

    @overload
    def __init__(self, x: float) -> None:
        ...

    def __init__(self, x):
        return x

# example2.py
from example import Foo


class Bar(Foo):
    pass

# index.rst
.. automodule:: example2
   :members:
   :undoc-members:

Expected behavior
The constructor of Bar class should be represented as two types.

Your project
No

Screenshots
No

Environment info

• OS: Mac
• Python version: 3.8.2
• Sphinx version: 3.1.0dev
• Sphinx extensions: sphinx.ext.autodoc
• Extra tools: No

Additional context
No