I believe this one is exactly what you're looking for,
but I don't know how well it works.   It wasn't on my
website until I just now re-uploaded it, which may mean
I found it inadequate at some point  .... or it may just mean
that I took it down as part of housekeeping at some point!

http://ockhamsbungalow.com/Python/Periodizer.zip

If it doesn't work, try Sparsifier.  I use it often and
know it works for the same general purpose, though
it doesn't have a specific "every nth keyframe" setting.

http://ockhamsbungalow.com/Python/Sparsifier2.zip

My python page
My ShareCG freebies