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.