Deprecation warning of np.str and np.object

[seisman]

Running the tests gives the following warnings:

DeprecationWarning: `np.str` is a deprecated alias for the builtin `str`. 
To silence this warning, use `str` by itself. 
Doing this will not modify any behavior and is safe. 
If you specifically wanted the numpy scalar type, use `np.str_` here.
Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
    @pytest.mark.parametrize("dtype", [np.str, np.object])

DeprecationWarning: `np.object` is a deprecated alias for the builtin `object`. 
To silence this warning, use `object` by itself.
Doing this will not modify any behavior and is safe. 
Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations
    @pytest.mark.parametrize("dtype", [str, np.object])

We may need to replace np.str with str, np.object with object.