rework dev dependencies (#620)

[willkg]

Previously, we maintained dev dependencies in a .in file and then used pip-compile to generate a requirements file and then used that for building dev environments and CI test environments.

We hit dependencies that have different dependencies depending on Python version and interpreter. That breaks with the compiled requirements method.

This redoes dev requirements so that they're an extra specified in setup.py. We don't need to track dependencies of dependencies or hash dev dependencies. We can update dev dependencies as we need to. Having stable pinned versions reduces unexpected errors because the universe changed.

Developers will need to do:

pip install -e '.[dev]'

now to pick up dev dependencies.

[willkg]

I updated the branch requirements and removed the checks that no longer get run and added the new pypy checks.

[willkg]

\o/ victory!

[willkg]

@g-k How does this look? Are there other bits of documentation that talk about the old way to install dev dependencies that I missed?

[willkg]

Thank you!