Add configuration for mypy

This does not require annotations yet, but it will check all code
outside of functions.

mypy.ini

@@ -0,0 +1,16 @@
+[mypy]
+implicit_reexport=False
+pretty=True
+show_error_codes=True
+strict_equality=True
+warn_redundant_casts=True
+warn_unreachable=True
+warn_unused_ignores=True
+
+mypy_path = $MYPY_CONFIG_FILE_DIR
+
+plugins =
+    mypy_django_plugin.main
+
+[mypy.plugins.django-stubs]
+django_settings_module = "tests.settings"