Make Mapping covariant.

Fixes #510.

This requires a `# type: ignore` on `__getitem__` and `get` because
mypy complains about covariant parameters.

FWIW typing.py needs to be changed too.  (It was covariant in the
value but invariant in the key -- typeshed was invariant in both.)

stdlib/2.7/typing.pyi

@@ -176,17 +176,19 @@ class ValuesView(MappingView, Iterable[_VT_co], Generic[_VT_co]):
     def __contains__(self, o: object) -> bool: ...
     def __iter__(self) -> Iterator[_VT_co]: ...
 
-class Mapping(Sized, Iterable[_KT], Container[_KT], Generic[_KT, _VT]):
-    @abstractmethod
-    def __getitem__(self, k: _KT) -> _VT: ...
+class Mapping(Sized, Iterable[_KT_co], Container[_KT_co], Generic[_KT_co, _VT_co]):
+    @abstractmethod  # type: ignore
+    def __getitem__(self, k: _KT_co) -> _VT_co:  # type: ignore
+        ...
     # Mixin methods
-    def get(self, k: _KT, default: _VT = ...) -> _VT: ...
-    def keys(self) -> list[_KT]: ...
-    def values(self) -> list[_VT]: ...
-    def items(self) -> list[Tuple[_KT, _VT]]: ...
-    def iterkeys(self) -> Iterator[_KT]: ...
-    def itervalues(self) -> Iterator[_VT]: ...
-    def iteritems(self) -> Iterator[Tuple[_KT, _VT]]: ...
+    def get(self, k: _KT_co, default: _VT_co = ...) -> _VT_co:  # type: ignore
+        ...
+    def keys(self) -> list[_KT_co]: ...
+    def values(self) -> list[_VT_co]: ...
+    def items(self) -> list[Tuple[_KT_co, _VT_co]]: ...
+    def iterkeys(self) -> Iterator[_KT_co]: ...
+    def itervalues(self) -> Iterator[_VT_co]: ...
+    def iteritems(self) -> Iterator[Tuple[_KT_co, _VT_co]]: ...
     def __contains__(self, o: object) -> bool: ...
 
 class MutableMapping(Mapping[_KT, _VT], Generic[_KT, _VT]):

stdlib/3/typing.pyi

@@ -226,16 +226,18 @@ class ValuesView(MappingView, Iterable[_VT_co], Generic[_VT_co]):
 
 # TODO: ContextManager (only if contextlib.AbstractContextManager exists)
 
-class Mapping(Iterable[_KT], Container[_KT], Sized, Generic[_KT, _VT]):
+class Mapping(Iterable[_KT_co], Container[_KT_co], Sized, Generic[_KT_co, _VT_co]):
     # TODO: Value type should be covariant, but currently we can't give a good signature for
     #   get if this is the case.
-    @abstractmethod
-    def __getitem__(self, k: _KT) -> _VT: ...
+    @abstractmethod  # type: ignore
+    def __getitem__(self, k: _KT_co) -> _VT_co:  # type: ignore
+        ...
     # Mixin methods
-    def get(self, k: _KT, default: _VT = ...) -> _VT: ...
-    def items(self) -> AbstractSet[Tuple[_KT, _VT]]: ...
-    def keys(self) -> AbstractSet[_KT]: ...
-    def values(self) -> ValuesView[_VT]: ...
+    def get(self, k: _KT_co, default: _VT_co = ...) -> _VT_co:  # type: ignore
+        ...
+    def items(self) -> AbstractSet[Tuple[_KT_co, _VT_co]]: ...
+    def keys(self) -> AbstractSet[_KT_co]: ...
+    def values(self) -> ValuesView[_VT_co]: ...
     def __contains__(self, o: object) -> bool: ...
 
 class MutableMapping(Mapping[_KT, _VT], Generic[_KT, _VT]):