Let MutableMapping.pop take a different default type. (#1044)

This is particularly useful for the idiom `d.pop(k, None)` to remove an item if it exists.

Fixes #278

Author: euresti

stdlib/2/__builtin__.pyi

@@ -558,7 +558,6 @@ class dict(MutableMapping[_KT, _VT], Generic[_KT, _VT]):
     def has_key(self, k: _KT) -> bool: ...
     def clear(self) -> None: ...
     def copy(self) -> Dict[_KT, _VT]: ...
-    def pop(self, k: _KT, default: _VT = ...) -> _VT: ...
     def popitem(self) -> Tuple[_KT, _VT]: ...
     def setdefault(self, k: _KT, default: _VT = ...) -> _VT: ...
     @overload

stdlib/2/typing.pyi

@@ -221,7 +221,7 @@ class MutableMapping(Mapping[_KT, _VT], Generic[_KT, _VT]):
     def __delitem__(self, v: _KT) -> None: ...
 
     def clear(self) -> None: ...
-    def pop(self, k: _KT, default: _VT = ...) -> _VT: ...
+    def pop(self, k: _KT, default: Union[_VT, _T] = ...) -> Union[_VT, _T]: ...
     def popitem(self) -> Tuple[_KT, _VT]: ...
     def setdefault(self, k: _KT, default: _VT = ...) -> _VT: ...
     @overload

stdlib/3/builtins.pyi

@@ -611,7 +611,6 @@ class dict(MutableMapping[_KT, _VT], Generic[_KT, _VT]):
     def __init__(self, iterable: Iterable[Tuple[_KT, _VT]], **kwargs: _VT) -> None: ...
     def clear(self) -> None: ...
     def copy(self) -> Dict[_KT, _VT]: ...
-    def pop(self, k: _KT, default: _VT = None) -> _VT: ...
     def popitem(self) -> Tuple[_KT, _VT]: ...
     def setdefault(self, k: _KT, default: _VT = None) -> _VT: ...
     @overload

stdlib/3/typing.pyi

@@ -296,7 +296,7 @@ class MutableMapping(Mapping[_KT, _VT], Generic[_KT, _VT]):
     def __delitem__(self, v: _KT) -> None: ...
 
     def clear(self) -> None: ...
-    def pop(self, k: _KT, default: _VT = ...) -> _VT: ...
+    def pop(self, k: _KT, default: Union[_VT, _T] = ...) -> Union[_VT, _T]: ...
     def popitem(self) -> Tuple[_KT, _VT]: ...
     def setdefault(self, k: _KT, default: _VT = ...) -> _VT: ...
     # 'update' used to take a Union, but using overloading is better.