Update _VideoArray base class to np.ndarray rather than npt.NDArray

[danielpmorton]

npt.NDArray as a base class seems to cause issues with numpy 2.5 and prevents importing of mediapy.

For instance, when I import mediapy with numpy 2.5 installed in my environment, I get

Traceback (most recent call last):
  File "mediapy/test.py", line 1, in <module>
    import mediapy
  File "mediapy/mediapy/__init__.py", line 1815, in <module>
    class _VideoArray(npt.NDArray[Any]):
TypeError: typealias() takes exactly 2 positional arguments (3 given)

Switching the base class to a standard array (rather than a type hint) fixes this problem. All test cases in mediapy_test still pass!