We have some dependencies that have recently or will soon break away

from any Python 2.7 support. This change introduces version range caps

on these in order to ensure we don't break (or continue to break).
The remaining dependencies that haven't been capped are still

advertising Python 2.7 support in the latest commits in their codebases,

or haven't been updated in many years, so there isn't a good raneg to

cap at right now.
The python-dateutil dependency has been removed in favor of the one

in Djblets.
rbintegrations has been capped to only work up to 2.x, so that we can

safely break compatibility in that series.
There's also an update to use the new autoprefix plugin dependency in

Djblets 1.0.15 dev.

Successfully installed in a new virtualenv.