change np.str to str to avoid warning

[CarlKCarlK]

bed_reader/bed_reader/tests/test_open_bed.py:628: 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
    val = np.zeros((3, 5), dtype=np.str)

[CarlKCarlK]

fixed with 0.2.8