[issue39558] Implement __len__() for itertools.combinations

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

This is essentially a duplicate of #27678 and #24849 (and there may be others).

Note that the math module now has `prod`, `comb` and `perm`, which match the lengths of the full `product`, `combinations` and `permutations` itertools functions.

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nosy: +mark.dickinson

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<https://bugs.python.org/issue39558>
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