> Perhaps by adding some new argument to the mmap constructor? (dup_fd = True)

I don't really like the idea of exposing the FD duplication to the
user, because:
- it's an implementation detail
- it doesn't reflect any argument of the POSIX mmap version
- it should False by default

I really think that application code expecting to be able to resize
the mmap after closing the FD is broken, but I'm also a really strong
advocate of backward compatibility...
I don't see any satisfying solution.