Put tobac on PyPI

[freemansw1]

We should probably put tobac on PyPI. This isn't urgent and it does make our release process more cumbersome, but it's important for non-conda users. The tutorial is here: https://packaging.python.org/en/latest/tutorials/packaging-projects/

[JuliaKukulies]

Good idea @freemansw1 and should probably not cause any problems given that all dependencies in requirements.txt exist on PyPI and given that we do not have any external dependencies.

I am not sure how much more cumbersome this will make our release process though. I have no experience with it but since there are many packages (e.g. MetPy) that are published on both the Anaconda cloud and PyPI, we can double check how they do it. I understand from this blog post that one can set the package location on PyPI as a source in the conda recipe, so that the package on conda-forge is automatically updated when the package is updated on PyPI. Not sure if it also works the other way around?