>
> What is the reason for mmap being a builtin module?

On Windows lots of modules are linked into the python main dll. The file
PC/config.c contains a list of all modules. From the point of the
maintainer it's much easier to link the modules into the main dll
instead of creating standalone dlls. I also suspect that it's much 

faster because relocation is slow (see PC/dllbase_nt.txt). Martin or
Mark can give you a better answer.

Why do you want to overwrite the existing module instead of using a
different name like ralfmmap?

import ralfmmap as mmap
sys.modules]'mmap'] = mmap