Instead of using Literal types, overload QuerySet.values_list in the plugin. Fixes #43. (#44)

- Add a couple of extra type checks that Django makes:
  1) 'flat' and 'named' can't be used together.
  2) 'flat' is not valid when values_list is called with more than one field.

mypy_django_plugin/main.py

@@ -229,6 +229,38 @@ def extract_proper_type_for_get_form(ctx: MethodContext) -> Type:
     return ctx.default_return_type
 
 
+def extract_proper_type_for_values_list(ctx: MethodContext) -> Type:
+    object_type = ctx.type
+    if not isinstance(object_type, Instance):
+        return ctx.default_return_type
+
+    flat = helpers.parse_bool(helpers.get_argument_by_name(ctx, 'flat'))
+    named = helpers.parse_bool(helpers.get_argument_by_name(ctx, 'named'))
+
+    ret = ctx.default_return_type
+
+    any_type = AnyType(TypeOfAny.implementation_artifact)
+    if named and flat:
+        ctx.api.fail("'flat' and 'named' can't be used together.", ctx.context)
+        return ret
+    elif named:
+        # TODO: Fill in namedtuple fields/types
+        row_arg = ctx.api.named_generic_type('typing.NamedTuple', [])
+    elif flat:
+        # TODO: Figure out row_arg type dependent on the argument passed in
+        if len(ctx.args[0]) > 1:
+            ctx.api.fail("'flat' is not valid when values_list is called with more than one field.", ctx.context)
+            return ret
+        row_arg = any_type
+    else:
+        # TODO: Figure out tuple argument types dependent on the arguments passed in
+        row_arg = ctx.api.named_generic_type('builtins.tuple', [any_type])
+
+    first_arg = ret.args[0] if len(ret.args) > 0 else any_type
+    new_type_args = [first_arg, row_arg]
+    return helpers.reparametrize_instance(ret, new_type_args)
+
+
 class DjangoPlugin(Plugin):
     def __init__(self, options: Options) -> None:
         super().__init__(options)
@@ -318,6 +350,11 @@ class DjangoPlugin(Plugin):
             if sym and isinstance(sym.node, TypeInfo) and sym.node.has_base(helpers.FORM_MIXIN_CLASS_FULLNAME):
                 return extract_proper_type_for_get_form
 
+        if method_name == 'values_list':
+            sym = self.lookup_fully_qualified(class_name)
+            if sym and isinstance(sym.node, TypeInfo) and sym.node.has_base(helpers.QUERYSET_CLASS_FULLNAME):
+                return extract_proper_type_for_values_list
+
         if fullname in {'django.apps.registry.Apps.get_model',
                         'django.db.migrations.state.StateApps.get_model'}:
             return determine_model_cls_from_string_for_migrations