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]

setup.cfg

@@ -19,3 +19,35 @@ exclude = jedi/third_party/* .tox/*
 
 [pycodestyle]
 max-line-length = 100
+
+
+[mypy]
+# Ensure generics are explicit about what they are (e.g: `List[str]` rather than
+# just `List`)
+disallow_any_generics = True
+
+disallow_subclassing_any = True
+
+# Ensure that optional types are explicit
+no_implicit_optional = True
+strict_optional = True
+
+# Avoid creating future gotchas emerging from bad typing
+warn_redundant_casts = True
+warn_unused_ignores = True
+warn_return_any = True
+warn_unused_configs = True
+
+warn_unreachable = True
+
+# Require values to be explicitly re-exported; this makes things easier for
+# Flake8 too and avoids accidentally importing thing from the "wrong" place
+# (which helps avoid circular imports)
+implicit_reexport = False
+
+strict_equality = True
+
+[mypy-jedi.debug]
+# jedi.debug is configured by setting module-level values, which mypy doesn't
+# know about.
+warn_unreachable = False

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',