Add type checking

In this case I have gone with pytype because in general I like their
approach, but mypy works fine as well. The main upside of mypy would be
that we would not be stuck using python3.7 for this, which is very
unfortunate.

Author: pganssle

.gitignore

@@ -17,6 +17,7 @@ docs/_build/
 .mypy_cache
 .pytest_cache
 .tox
+.pytype
 
 # Temporary files
 tmp/

tox.ini

@@ -12,6 +12,15 @@ deps =
 commands =
     python update.py {posargs}
 
+[testenv:pytype]
+description = Run typechecking
+basepython = python3.7
+skip_install = True
+deps =
+    pytype
+commands =
+    pytype {posargs} update.py
+
 [testenv:format]
 description = Run auto formatters
 skip_install = True