Support other Python packaging formats

[woodruffw]

Outside of requirements.txt, there are a few other common Python packaging files:

• A few different tools store requirements in pyproject.toml (Support pyproject.toml #83)
• poetry puts locked (i.e., frozen) dependencies in poetry.lock (Support Poetry via poetry.lock #84)
• pipenv uses Pipfile and Pipfile.lock (Support pipenv via Pipfile and Pipfile.lock #85)

Each of these functionally boils down to a RequirementsSource, but with a bit of pre-processing to get them out of their dedicated formats.

[woodruffw]

This was in the original SoW but indicated that it wasn't high in priority, so I'm marking it as potentially follow-on for now.

[di]

One slight complication for poetry here is that it currently requires packaging<21.0,>=20.4 whereas we require:

https://github.com/trailofbits/pip-audit/blob/cd4dd0825fcff3153d7bcd8ea320dc6f13377f14/setup.py#L32

[di]

Let's split this into separate issues for each of these.

[woodruffw]

Broken out, I turned this issue into a checkbox tracker for each sub-issue.

[woodruffw]

Closing in favor of subissues, since we don't really need a roadmap-type tracker here.