Initial typing support

This enables type checking in CI and marks the project as typed.

tox.ini

@@ -2,6 +2,7 @@
 envlist =
     py{39,38,37,36,py3}
     style
+    typing
     docs
 skip_missing_interpreters = true
 
@@ -24,6 +25,10 @@ deps = pre-commit
 skip_install = true
 commands = pre-commit run --all-files --show-diff-on-failure
 
+[testenv:typing]
+deps = -r requirements/typing.txt
+commands = mypy
+
 [testenv:docs]
 deps =
     -r requirements/docs.txt