Most of our command line tools were updated to allow Djblets to patch

the collections module on Python 3.10, but rbext was not. This meant

that if you're using Python 3.10, you're unable to invoke rbext at all

on Python 3.10. Whether this has been hit in the wild is unknown, but

it's blocking a new CI pipeline.
We now import djblets early in the module, giving Djblets the

opportunity to apply the patch.

Successfully ran rbext using Python 3.10.