src/rank_filter.pyx: Drop `itertools` alias.
src/rank_filter.pyx: Drop `itertools` alias.

jakirkham/rank_filter,nanshe-org/rank_filter,jakirkham/rank_filter,nanshe-org/rank_filter,DudLab/rank_filter,nanshe-org/rank_filter,DudLab/rank_filter,jakirkham/rank_filter,DudLab/rank_filter