Ability to directly spawn random number generators
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`numpy.random.Generator.spawn` now allows to directly spawn new
independent child generators via the `numpy.random.SeedSequence.spawn`
mechanism.
`numpy.random.BitGenerator.spawn` does the same for the underlying
bit generator.

Additionally, `numpy.random.BitGenerator.seed_seq` now gives direct
access to the seed sequence used for initializing the bit generator.
This allows for example::

    seed = 0x2e09b90939db40c400f8f22dae617151
    rng = np.random.default_rng(seed)
    child_rng1, child_rng2 = rng.spawn(2)

    # safely use rng, child_rng1, and child_rng2

Previously, this was hard to do without passing the ``SeedSequence``
explicitly.  Please see `numpy.random.SeedSequence` for more information.
