Initial typing support

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

setup.cfg

@@ -85,3 +85,30 @@ max-line-length = 80
 per-file-ignores =
     # __init__ module exports names
     src/flask/__init__.py: F401
+
+[mypy]
+files = src/flask
+python_version = 3.6
+allow_redefinition = True
+disallow_subclassing_any = True
+# disallow_untyped_calls = True
+# disallow_untyped_defs = True
+# disallow_incomplete_defs = True
+no_implicit_optional = True
+local_partial_types = True
+# no_implicit_reexport = True
+strict_equality = True
+warn_redundant_casts = True
+warn_unused_configs = True
+warn_unused_ignores = True
+# warn_return_any = True
+# warn_unreachable = True
+
+[mypy-asgiref.*]
+ignore_missing_imports = True
+
+[mypy-blinker.*]
+ignore_missing_imports = True
+
+[mypy-dotenv.*]
+ignore_missing_imports = True