Use a separate local tox env to test with local dependencies

This simplifies the dev setup by making it possible to work on tasks in complete isolation, without even an Arkindex architecture installed, and no other repositories. This means that neither tasks, nor transkribus-client, nor api-client, nor Ponos, nor teklia-toolbox need each other to work.

Running tox will use the PyPI packages as mentioned in requirements.txt, and running tox -e local will instead use ../api-client and ../transkribus.

I updated the .gitlab-ci.yml so that only the local mode is used with the two clones from master, but I can make the CI use both environments if needed.