[issue35431] Add a function for computing binomial coefficients to the math module

[kellerfuchs]

kellerfuchs <kellerfuchs at hashbang.sh> added the comment:

So, I rebased Yash's and my branches, and merged them together.  The result is still in PR#11018.

This involved a few changes, which seem to reflect the consensus here:
- raise ValueError if k>n ;
- rename the function to math.combinations.

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<https://bugs.python.org/issue35431>
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