ENH: Make type hinting compatible with Python 3.9

Make type hinting compatible with Python 3.9: use `Union` instead of the
pipe operator `|` to support type hints in Python 3.9. The pipe operator
was overloaded for writing union types starting Python 3.10:
https://peps.python.org/pep-0604/
https://docs.python.org/3.9/library/typing.html
https://docs.python.org/3.10/library/typing.html

nireports/tools/ndimage.py

@@ -40,7 +40,7 @@
 Mat = npt.NDArray[np.float64]
 
 
-def rotation2canonical(img: SpatialImage) -> Mat | None:
+def rotation2canonical(img: SpatialImage) -> ty.Union[Mat | None]:
     """Calculate the rotation w.r.t. cardinal axes of input image."""
     img = nb.as_closest_canonical(img)
     # XXX: SpatialImage.affine needs to be typed