TYP: fix np.random.{get,set}_bit_generator implicit re-exports (#32257) - #32291

Backport of #32257.

At runtime, np.random re-exports get_bit_generator, yet it's not in np.random.__all__:

>>> import numpy as np
>>> np.random.get_bit_generator()
<numpy.random._mt19937.MT19937 object at 0x12ab85a0>
>>> "get_bit_generator" in np.random.__all__
False

The way to express this in the stubs is by using a redundant symbol alias (viz. import x as x), as specified in the Python typing spec: https://typing.python.org/en/latest/spec/distributing.html#import-conventions
Without this redundant alias (the current sitiation) accessing np.random.get_bit_generator will cause checkers to report an error:

No AI.