Python: Extracting every nth m items from a list : Champs,

What's the most efficient way of extracting every nth m items from a list. So if I have: 
[1,2,3,4,5,6,7,8,9,10,...]
I would like every 3rd 2 items, starting from the first index:
[1,2,7,8,13,14,...]

Thanks
Kam 