Move to an `src/` layout

[pradyunsg]

I got very confused a few minutes ago, because I had a local copy of packaging that was not able to parse requests == 2.8.* and that's obviously wrong. However, pip list only found the installed copy of packaging, and not the local copy of it.

[brettcannon]

I'm still not a massive fan, but I also realize I'm in the minority, so I won't stop you if you make the change.

[hauntsaninja]

It looks like this happened in #626

[pradyunsg]

Indeed!