stacked singledispatches use wrong type annotation

[anntzer]

Describe the bug

When multiple @functools.singledispatch decorators are stacked (as documented by the stdlib docs at https://docs.python.org/3/library/functools.html#functools.singledispatch), the type annotations reported by sphinx are incorrect.

To Reproduce
Steps to reproduce the behavior:

cat >project.py <<EOF
from functools import singledispatch

@singledispatch
def foo(x): "something"

@foo.register(int)
@foo.register(float)
def _(x): "a number"
EOF
sphinx-apidoc . -o . -F -A me -V 0.0
PYTHONPATH=. make html

then open _build/html/project.html: you get

project.foo(x)                 [source]
project.foo(x: float)
project.foo(x: float)
something

Expected behavior
The correct overloads are

project.foo(x)
project.foo(x: int)
project.foo(x: float)

Your project
N/A

Screenshots
N/A

Environment info

• OS: Arch Linux
• Python version: 3.9.1
• Sphinx version: 3.4.3
• Sphinx extensions: sphinx.ext.autodoc
• Extra tools: N/A

Additional context
N/A