User-facing type annotations don't work for to_Vector*D methods

^{Originally posted by acampove September 22, 2025 in #640}

This is the MWE

import vector
from vector import MomentumObject3D

def _fun(vec : MomentumObject3D) -> None:
    print(vec)

vec = vector.obj(pt=1.0, eta=1.0, phi=1.0, mass=10)
v3d = vec.to_Vector3D()

_fun(vec=v3d)

save it, run pyright/mypy test.py and you will see:

(rx) [acampove@thinkbook vector]$ pyright test.py
WARNING: there is a new pyright version available (v1.1.404 -> v1.1.405).
Please install the new version or set PYRIGHT_PYTHON_FORCE_VERSION to `latest`

/home/acampove/Tests/vector/test.py
  /home/acampove/Tests/vector/test.py:10:10 - error: Argument of type "VectorProtocolSpatial" cannot be assigned to parameter "vec" of type "MomentumObject3D" in function "_fun"
    "VectorProtocolSpatial" is not assignable to "MomentumObject3D" (reportArgumentType)
1 error, 0 warnings, 0 informations

Start with the type definitions behind vector.obj and its to_Vector3D method, then reproduce the issue from the Python example using pyright or mypy. Done means the returned value is accepted as MomentumObject3D in the shown _fun call without the reported type error.