Change RandomState to default_rng

[mgeier]

This is used in sfs.array.linear_random().

IIUC, this has been changed in NumPy 1.17, so we should probably wait a while before we are doing this.

See:

• https://docs.scipy.org/doc/numpy/reference/random/legacy.html
• https://docs.scipy.org/doc/numpy/reference/random/generator.html