Don't specify dev requirements in both .in files

[sandypreiss]

We currently pin development requirements in both requirements.in and requirements_ci.in. This is duplicative, and can be avoided by using pip constraints. The solution would involve building one of the requirement.txt files, then using that as a constraint in the other .in file. Then when pip sees black in the second .in file, it will also see the constraint and use that version. Better explanation with example at https://github.com/jazzband/pip-tools#workflow-for-layered-requirements