restricted types of itertools stubs
fixed incorrect restrictions
fixed output of product to Iterator[Sequence[T]]
fixed input type of product, and added note about type sig
Fix typo in encodings stubs

Fixes #369.
Fix importing mypy.codec.pytokenize in Python 3

Closes #370.
Move mypy/codec/example.py to samples/codec/example.py

The custom coding was causing all kinds of trouble during installation.
Merge pull request #358 from mvcisback/itertools

Restrict types for stubs in itertools