Add mypy to lint checks

We already have a few type annotations, it makes sense to lint with
mypy.

Author: lkollar

test-requirements.txt

@@ -6,3 +6,4 @@ flake8
 pretend
 docker
 pyelftools
+mypy

tox.ini

@@ -10,6 +10,7 @@ commands =  pytest --doctest-modules auditwheel tests []
 
 [testenv:lint]
 commands = flake8 auditwheel
+           mypy auditwheel
 
 [testenv:cov]
 commands = python -m pytest tests/unit --cov=auditwheel --cov-report=term-missing