Specifying self-type for first init overload affects isinstance type narrowing

The task

I have a [generic class with multiple `__init__` overloads](https://github.com/ni/nitypes-python/blob/01cd89bfa4f33e8ed88eae0fe495715059653b8f/src/nitypes/waveform/_spectrum.py#L278 ). One of the `__init__` overloads specifies the type of `self` in order to control what the type variable evaluates to for this overload.