setup: require specific versions of dependencies, both at run-time (if pkg_resources...

setup: require specific versions of dependencies, both at run-time (if pkg_resources is available) and at build-time, and make there be only once place where we specify those versions
Using pkg_resources.require() like this also apparently allows people to install multiple different versions of packages on their system and tahoe (if pkg_resources is available to it) will import the version of the package that it requires.  I haven't tested this feature.