Drop requirements.txt

[mrocklin]

Currently this package has a requirements file that includes packages like pytest and pip. Probably these aren't actually requirements, but instead used for development.

I was confused for a moment and thought that these were actual requirements, but checked setup.py and setup.cfg and found that they weren't mentioned, so maybe this isn't an issue.

Maybe these should be renamed to requirements-dev.txt or something for clarity?

Not a big deal though, I was just briefly confused and thought I'd raise an issue. Please feel free to ignore.