To check the CLI and client library can work without any dependency
on Django and friends.
This requires a lot of moving code around.
In particular, I had to move the content of the toplevel conftest.py
to swh/deposit/tests/conftest.py or pytest would error with
ImportPathMismatchError issues when running the py3-clientonly env
(and PY_IGNORE_IMPORTMISMATCH=1 merely replaced this error with other
errors about not finding the fixtures when running tests)