[isort](https://isort.readthedocs.io/en/latest/) sorts your Python
imports so you don't have to. This makes sure that imports are always
where they should be and prevents issues like duplicated imports and
merge conflicts. Using pre-commit, this can be done automatically
without any manual steps.
Depends on #9.
[pre-commit](https://pre-commit.com/) automatically runs tools like
linters and fixers to make development easier. This change adds a simple
pre-commit config file with an example trailing whitespace fixer and
applies it to the existing project.
Additionally, don't ignore the lib directory (an error introduced in #8).
