Fix numpy str deprecations

[thomashopf]

in sequences_to_matrix
matrix = np.empty((N, L), dtype=np.str)
AttributeError: module 'numpy' has no attribute 'str'.
np.str was a deprecated alias for the builtin str. To avoid this error in existing code, 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.
The aliases was originally deprecated in NumPy 1.20; for more details and guidance see the original release note at:
https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations

[charlesxu90]

Met with a same issue when install in python 3.10. Any solutions?

BTW, the code is working well in python 3.7.