Move library into src/ directory

[nickjj]

After building this locally I noticed all of the expected files were in
the wheel where as before this patch it was missing a lot of files.

This idea of using a src/ directory is mentioned in the official Python
documentation for packaging files at:

https://packaging.python.org/en/latest/tutorials/packaging-projects/

It's also used in Flask and other large Python projects.

I don't have access to push this to the test PyPi repo to give it a whirl
so I'm not 100% sure if it "really" works.

If this does end up working, I think the order of operations would be:

• Push it to the test PyPi repo
• Install the package into a real project
• Confirm it works
• Modify the version to be 0.13 instead of 0.13.dev0
• Build the release
• Push it to the real PyPi repo
• Install the package into a real project
• Confirm it works
• Update the changelog
• Git tag the release

[jeffwidman]

Yeah, this makes a lot of sense and src is definitely an idiomatic location.