Fix check on RandomState type to use isinstance

[frankier]

Currently if we try and pass in a RandomState to e.g. make_classification(...) like so:

from cuml.datasets import make_classification
from cupy.random import RandomState
rng = RandomState(42)
x, y = make_classification(40, 10, random_state=rng)

then we get:

  File "/opt/conda/lib/python3.8/site-packages/cuml/internals/api_decorators.py", line 473, in inner
    ret_val = func(*args, **kwargs)
  File "/opt/conda/lib/python3.8/site-packages/cuml/datasets/classification.py", line 210, in make_classification
    generator = _create_rs_generator(random_state)
  File "/opt/conda/lib/python3.8/site-packages/cuml/datasets/utils.py", line 39, in _create_rs_generator
    raise ValueError('random_state type must be int or CuPy RandomState')
ValueError: random_state type must be int or CuPy RandomState

This PR replaces the current type detection approach with a more conventional isinstance(...) check, fixing the problem.

[GPUtester]

Can one of the admins verify this patch?

[JohnZed]

ok to test

[JohnZed]

You'll need to update the copyright header to include 2021 for CI to run here.

[JohnZed]

rerun tests

[JohnZed]

Thank you for this implementation! Ended up adding a slightly-different version of your same idea in #3716