Bug 1460470 - Make run-task somewhat usable on Python 3; r=mshal

This required a lot of attention to bytes versus strings.

The hacks around handling process output are somewhat gross. Apparently
readline() doesn't work on bytes streams in Python 3?! So we install a
custom stream decoder so we can have nice things.

There are still some failures in run-task on Python 3. But we're a big
step closer.

MozReview-Commit-ID: 4FJlTn3q9Ai