Split development dependencies from runtime dependencies

[jkeifer]

I was noticing that pytest was getting pulled in to a project's runtime dependencies per it's dependency on this package. Looking more closely at the code, it appears types-setuptools and pytest are not actual runtime dependencies of this package but development-only dependencies.

This PR is to split those two packages into a extra requirements set called dev so they aren't part of the runtime dependency set for this package.

Now, for local development, instead of running pip install -e ., one would run pip install -e '.[dev]'.

[jkeifer]

The tests are failing, but that looks to be a pre-existing issue I'm not sure how to address at the moment.

[jonhealy1]

The tests are failing, but that looks to be a pre-existing issue I'm not sure how to address at the moment.

This looks great. I will try to figure out the tests situation. Thanks