evolve: use more often seen metavariables in command synopsis strings Also use ellipsis when multiple options are acceptable.

topic: add more options to command synopsis string

debian: also build with python3 We build with Python 3 along with Python 2. The binary package will contain the extension for both python version. This is to ease transition until the mercurial package in Debian uses Python 3. The documentation is still built with Python 2 because it needs Mercurial (which is currently Python 2).