CLN: Use cython algo for groupby var with ddof != 1
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cc @rhshadrach 

I think you've implemented ddof on the cython level a while back. Any reasons why var is taking a different path? Results look sensible and no failing tests

Also, gives a nice speedup 
We've got the numba tests that are testing consistency between numba and cython, so I think we are good. They are testing ddof 0 Thanks @phofl 