[build] Audit dependency versions #950
Description
There are some places where our dependencies have fallen out of date, which has recently led to a few mad scrambles to fix CI when it has broken unexpectedly. (See the various TODOs in https://github.com/getsentry/sentry-python/blob/master/tox.ini.)
It'd be great if we could undertake a general audit of our dependency versions, to see if we can't bring more of them up to current.