Fix mypy errors caused by new version of mypy

[jabraham17]

Fixes mypy errors caused by new versions of mypy, which showed up as:

arkouda/numpy/pdarraysetops.py:519: error: Item "Strings" of "pdarray | Strings | Categorical" has no attribute "max_bits"  [union-attr]
arkouda/numpy/pdarraysetops.py:519: error: Item "Categorical" of "pdarray | Strings | Categorical" has no attribute "max_bits"  [union-attr]
arkouda/numpy/pdarraysetops.py:520: error: Item "Strings" of "pdarray | Strings | Categorical" has no attribute "max_bits"  [union-attr]
arkouda/numpy/pdarraysetops.py:520: error: Item "Categorical" of "pdarray | Strings | Categorical" has no attribute "max_bits"  [union-attr]
arkouda/numpy/pdarraysetops.py:525: error: Item "Strings" of "pdarray | Strings | Categorical" has no attribute "max_bits"  [union-attr]
arkouda/numpy/pdarraysetops.py:525: error: Item "Categorical" of "pdarray | Strings | Categorical" has no attribute "max_bits"  [union-attr]

[bradcray]

Looks good to me, thanks! Note that I edited the OP to capture the mypy errors that these changes address.