update os.sendfile() doc signaling that cross-platform apps should not use headers, trailers and flags arguments
Issue #21346: Fix typo, make message consistent in test_itertools.

Pointed out by Brian Kearns.
Closes #21346: Merge with 3.4
Issue8297: module attribute lookup failures now include module name in error message.
do not override errors from descriptors on modules
sprinkle some _PyId goodness around moduleobject.c