Deprecation warning from use of numpy.bool

[marcpaterno]

Running pytest -v tests/ when using the most recent numpy generates a deprecation warning:

test/test_sacc2.py: 10 warnings
  /Users/paterno/repos/sacc/sacc/sacc.py:301: DeprecationWarning: `np.bool` is a deprecated alias for the builtin `bool`. To silence this warning, use `bool` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.bool_` here.
  Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
    if indices.dtype != np.bool:

This can be fixed as given in the warning message if we move to numpy 1.20 as a requirement. Since support for numpy 1.19 has already ended this seems like a good solution.