Configure mypy

No fixes yet, this just gets the config in place.

Note: I'm assuming that we'll pick up a change to parso such that
it exposes its type stubs here. Otherwise we'll want to tweak the
imports config to ignore those errors.

.travis.yml

@@ -32,6 +32,7 @@ matrix:
       script:
         # Ignore F401, which are unused imports. flake8 is a primitive tool and is sometimes wrong.
         - 'flake8 --extend-ignore F401 jedi setup.py'
+        - 'mypy jedi'
 install:
     - sudo apt-get -y install python3-venv
     - pip install .[testing]