This type is something core to Python and is useful when typing web applications,
but doesn't actually exist in the stdlib anywhere. I put this in wsgiref, but I am
open to suggestions as for a better place.
(Original PR by @rowillia.)
1. Mentioned review requirement for core contributors.
2. Made stub coding style on par with requirements specified on
the Mypy wiki (we can merge them now).
Fixes#720.
Related changes: used a NamedTuple for time.struct_time on Python 3, used
sys.version selectors for proper typing of #716, added missing *Style classes
to logging on Python 3.
Łukasz Langa