fix: make numpy.typing import explicit

[xyf5432]

Fixes #45

Problem

np.typing.XXX is only accessible on numpy < 2.0 if numpy.typing has been explicitly imported earlier. Without a version constraint forcing numpy >= 2, users with numpy 1.x installed can hit:

AttributeError: module 'numpy' has no attribute 'typing'

when the annotation is evaluated at import time.

Fix

File: mjinx/typing.py

  from __future__ import annotations
+ import numpy.typing
  import jax.typing

  ArrayLike: TypeAlias = np.typing.ArrayLike | jax.typing.ArrayLike

(ArrayLike: TypeAlias = ... is a module-level expression — PEP 563 does not defer it. Today the alias only resolves through an incidental side effect: trimesh, pulled in via mujoco-mjx, imports numpy.typing at startup. The explicit import removes that hidden reliance.)

Verification

• Verified in a real environment (numpy 1.26.4 + jax 0.7.0 + mujoco 3.11.0 + mujoco-mjx 3.11.0): from mjinx import typing succeeds today because trimesh (a transitive dependency via mujoco-mjx) executes import numpy.typing at import time — line 31's np.typing.ArrayLike only resolves through that side effect.
• With the explicit import, the ArrayLike alias no longer depends on trimesh's internals or the import order inside mujoco-mjx — if that transitive chain changes in the future, this module keeps working.
• The change only adds an import — no behavioral change.

[xyf5432]

I dug into the three failing CI checks and pushed fixes (commits b4c7482, 7239283, d60c179). All were pre-existing issues, unrelated to the one-line import numpy.typing change:

1. Unit tests — AttributeError: 'mujoco._structs.MjModel' object has no attribute 'impl' in test_get_distance_with_hfield. Root cause: mjx.kinematics() expects the mjx wrapper types (mujoco.mjx.Model/Data, which expose .impl), but the test passes the raw mujoco.MjModel. This is version-independent (the .impl reference exists in mujoco_mjx 3.7+), so pinning mujoco was not a fix. The test now wraps the model with mjx.put_model() before calling kinematics — verified the full unittest suite passes.
2. test (3.10) — pytest 9.1.1 rejects the single-string filterwarnings in pyproject.toml at startup. Changed it to the standard list form (accepted by pytest 8 and 9) — pytest tests/example_tests now runs and passes (4 passed).
3. mypy — two Incompatible types in assignment errors: jax.Array is not a collections.abc.Sequence, so assigning jnp.zeros(...) to the Sequence-typed setters failed. Widened the setter signatures to jnp.ndarray | Sequence (the setters already convert via jnp.array()) — mypy mjinx now reports no issues.

All three checks were verified locally on the same dependency versions CI resolves (mujoco 3.12.0, pytest 9.1.1, jax 0.11.1).

[domrachev03]

Thank you for addressing them! Looks good to me