Revisit pinned dependencies

[ANich]

In https://github.com/mozilla/python_mozetl/pull/379, we pinned a few dependencies as CI workarounds (flake8 and protobuf). There are also a couple other dependencies pinned (see https://github.com/mozilla/python_mozetl/pull/374 and https://github.com/mozilla/python_mozetl/pull/377) that could be bumped.

For the most part, this repo isn't being actively developed so this is probably fine but opening this issue for if/when we revisit dependencies more generally.