explicitly mark value and label types on choices enum (typeddjango#406)

In addition to the meta properties added
to the Choices enums, value and label are
set for each choice.
This commit explicitly types those instead of the
tuple value and non-extant label types

django-stubs/db/models/enums.pyi

@@ -10,6 +10,10 @@ class ChoicesMeta(enum.EnumMeta):
 
 class Choices(enum.Enum, metaclass=ChoicesMeta):
     def __str__(self): ...
+    @property
+    def label(self) -> str: ...
+    @property
+    def value(self) -> Any: ...
 
 # fake
 class _IntegerChoicesMeta(ChoicesMeta):
@@ -18,7 +22,9 @@ class _IntegerChoicesMeta(ChoicesMeta):
     labels: List[str] = ...
     values: List[int] = ...
 
-class IntegerChoices(int, Choices, metaclass=_IntegerChoicesMeta): ...
+class IntegerChoices(int, Choices, metaclass=_IntegerChoicesMeta):
+    @property
+    def value(self) -> int: ...
 
 # fake
 class _TextChoicesMeta(ChoicesMeta):
@@ -27,4 +33,6 @@ class _TextChoicesMeta(ChoicesMeta):
     labels: List[str] = ...
     values: List[str] = ...
 
-class TextChoices(str, Choices, metaclass=_TextChoicesMeta): ...
+class TextChoices(str, Choices, metaclass=_TextChoicesMeta):
+    @property
+    def value(self) -> str: ...