Cleanup

* I remove the `TODO` file because it's old and I don't think accurate anymore.
* I remove the `MANIFEST.in` file and instead use `setuptools_git` -- this adds everything managed by git to the sdist. Which means that cruft that the person building the release has in their working directory won't be package and the stuff which is managed with git all will and no need to maintain a `MANIFEST.in`.

Cc: @ramiro, @damoxc, @rsyring