[flake8-pyi] Avoid false positives on singledispatch functions (PYI041) - #27335

…PYI041`)

Summary
--

Fixes #27333. The fixed annotation isn't actually equivalent in this case.

```pycon
>>> import functools
...
... @functools.singledispatch
... def int_or_float(value: object) -> str:
...     return False
...
... @int_or_float.register
... def _(value: int | float):
...     return True
...
... assert int_or_float(3)
...
>>> import functools
...
... @functools.singledispatch
... def int_or_float(value: object) -> str:
...     return False
...
... @int_or_float.register
... def _(value: int ���at):
...     return True
...
... assert int_or_float(3)
KeyboardInterrupt
>>> import functools
...
... @functools.singledispatch
... def int_or_float(value: object) -> str:
...     return False
...
... @int_or_float.register
... def _(value: float):
...     return True
...
... assert int_or_float(3)
...
Traceback (most recent call last):
  File "<python-input-1>", line 11, in <module>
    assert int_or_float(3)
           ~~~~~~~~~~~~^^^
AssertionError
```

We had an existing helper for testing for `singledispatch` functions, which we use to skip the first
parameter of such functions since this is the dispatching parameter.

Test Plan
--

New mdtests