Use isort

[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.

tests/__init__.py

@@ -1,2 +1,3 @@
 import sys
+
 sys.path.insert(1, '../lib/')