Make req-dev include req-tests and add requests explicitly to req-tests

[hategan]

This should address #283.

@wilke, can you try setting up the tests with this branch?

[jameshcorbett]

Looks reasonable to me, FWIW.

[wilke]

Command: ./psij-ci-run
Status: OK

Tests are running and i installed the requirements with python -m pip install --target .packages --upgrade -r requirements-dev.txt

There is one error message at build time:

ERROR: After October 2020 you may experience errors when installing or updating packages. This is because pip will change the way that it resolves dependency conflicts.

We recommend you use --use-feature=2020-resolver to test your packages with the new resolver before it becomes the default.

sphinx-tabs 3.4.1 requires docutils~=0.18.0, but you'll have docutils 0.17.1 which is incompatible.