Prefect packaging clean-up about prefect

[cicdw]

I'm going to add to this list:

• consider removing environment.yml from the repo, as we don't officially use it anywhere in CI, and as @jlowin mentioned in a previous comment (now hidden in the pre-github days), this creates the potential for .yml and requirements.txt mismatches

[cicdw]

A few notes for documentation purposes:

• tox doesn't add any benefit for us right now; in fact, to get tox set up / installed in CI requires an additional environment creation tool to get multiple base installs of python, and this seemed like more work than testing against multiple images in Circle as we currently do
• we need to make sure we always test a built version of the prefect package in CI, as opposed to testing directly off the repo (this precludes using pip install -e in CI). A standard pip install works fine for this purpose. This ensures that our MANIFEST.in and any other packaging-related things are actually tested