The new --all-pyvers argument allows a setup.py command to run for

every supported version of Python, simplifying the setup process

considerably.
prepare-dev.py has also gained this option, which it will pass on to

setup.py. This isn't the default because someone setting up a tree

might not have a multi-Python virtualenv set up, but it's available if

needed.

Tested both commands with and without --pyvers.