Add renovate.json and separate requirements for tox environments

[misohu]

Each tox environment has its own separate requirements-<env>.txt file generated with pip-tools from requirements-<env>.in file.

There is new tox environment update-requirements which will generate requirements-.txt from requirements-.in files.

I also added renovate.json for renovate setup. I unpinned versions for every package except flake (as flake-copyright is causing problems).