Achieving this buffer in Python

[StevenCurran]

Hi there Willem, I was taking a look at your repo on github and have been looking to do something similar in python. 

However, I cannot seem to figure out how to do this due to not having the pointer arithmetic available within the mmap call to python.  https://github.com/willemt/cbuffer/blob/master/cbuffer.c#L52

Line 52 is what I cannot mimic, because the offset param here does not help us with the window.  Any help would be greatly appreciated!   

Regards