Upgrade Apache Beam to use Cython 3.x.

[robertwb]

Cython 3 has been out for over a year now (with the first pre-release predating that by several years).

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[damccorm]

Thanks - this was on my backlog :)

[robertwb]

I think the Could not retrieve licences for packages ['scikit-learn'] errors are irrelevant.

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[tvalentyn]

@robertwb Does your recommendation in https://github.com/apache/beam/pull/27536/files#r1267273865 still apply?

[robertwb]

Good call. I did take a pass over the pxd files, but forgot about the nogil caveat. Looks like we don't use that much, but added the one case I found. (Most of the time we were either explicitly using except or returning a Python object, and even where not it's numeric so should be fast.)

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