buffer arguments need to be restricted

[dmoisset]

Some function arguments accept only objects with a buffer interface as described by PEP-3118. That's a C level API and it's not backed by any Python methods so it's hard to describe in mypy, so those arguments are currently described as Any

This need some support from python; see some related discussions at:

http://permalink.gmane.org/gmane.comp.python.devel/156411
http://bugs.python.org/issue27501