Add mypy to CI

Since we already have some type hints in the codebase let's make sure
they're actually consistent.

.travis.yml

@@ -17,4 +17,8 @@ matrix:
     - { python: "3.7", dist: xenial, sudo: true }
 install:
   - pip install pytest "typing$TYPING_VERSION" "pytest-cov>=2.5.1"
-script: py.test -vv --cov=injector --cov-branch --cov-report html --cov-report term
+  # mypy can't be installed on pypy
+  - if [[ "${TRAVIS_PYTHON_VERSION}" != "pypy"* ]] ; then pip install mypy ; fi
+script:
+  - py.test -vv --cov=injector --cov-branch --cov-report html --cov-report term
+  - if [[ "${TRAVIS_PYTHON_VERSION}" != "pypy"* ]] ; then mypy injector.py ; fi

mypy.ini

@@ -0,0 +1,7 @@
+[mypy]
+ignore_missing_imports = true
+follow_imports = error
+warn_no_return = true
+warn_redundant_casts = true
+# TODO: disallow untyped defs once we have full type hint coverage
+disallow_untyped_defs = false