Async Support For Prefork

This removes the deprecated function call_capture_stdout and
contains some whitespace tweaks.

Require at least six 1.8.0 (for six.moves.intern)

Since commit d855850c, function `six.moves.intern` is used in pytools,
but this function appeared only in six 1.8.0.
This commit prevents an error while importing pytools with an older six installed.

Fixed exception when importing decorator in python 2.7