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Previously when doing floor division numpy would sometimes return an incorrect
signed zero. For example:

>>> np.zeros(10)//1
array([-0., -0., -0., -0., -0., -0., -0., -0., -0., -0.])

>>> np.remainder(-1.0,1.0)
-0.0

The reason for this is that whenever div or mod were zero the code was using
the following to pick the sign of zero:

floordiv = (a / b > 0) ? 0.0@c@ : -0.0@c@;

This commit updates these lines to instead use the copysign function which is
how cpython does floor division.

Fixes #12841.

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BUG, DOC: test, fix that f2py.compile accepts str and bytes, rework docs

I'm sorry I didn't get this in earlier! Hopefully this will make it clearer to
users ``__array_function__`` is only opt-in for now.

ENH: Better links in documentation