Pin test requirements + Python version

[ahal]

I noticed that the test requirements are unpinned:
https://github.com/mozilla-releng/scriptworker/blob/main/tox.ini#L17

We should follow the conventions in other releng repos of using pip-compile-multi and a set of test.in / test.txt requirements files to keep the CI reproducible.

Relatedly, the version of Python we are using is also unpinned. This caused some confusion in:
#583