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.

setup.py

@@ -46,6 +46,7 @@ setup(name='jedi',
           ],
           'qa': [
               'flake8==3.8.3',
+              'mypy==0.782',
           ],
       },
       package_data={'jedi': ['*.pyi', 'third_party/typeshed/LICENSE',