Overload signature of get to return an Optional value and to allow default to take any type to match runtime behavior.

This chage more closely matches the behavior of `get` at runtime.  Users can pass whatever they want in to the default
parameter and it will be returned if the key is absent.  Additionally, `get` should return an `Optional` if called with
only one parameter.

```python
z = {'a': 22}
reveal_type(z.get('b'))
reveal_type(z.get('b', 22))
reveal_type(z.get('b', 'hello'))
```

Before:
```shell
test_get_default.py:2: error: Revealed type is 'builtins.int*'
test_get_default.py:3: error: Revealed type is 'builtins.int*'
test_get_default.py:4: error: Revealed type is 'builtins.int*'
test_get_default.py:4: error: Argument 2 to "get" of "dict" has incompatible type "str"; expected "int"
```

After:
```shell
test_get_default.py:2: error: Revealed type is 'Union[builtins.int*, builtins.None]'
test_get_default.py:3: error: Revealed type is 'builtins.int'
test_get_default.py:4: error: Revealed type is 'Union[builtins.int, builtins.str*]'
```

Author: Roy Williams

stdlib/2/__builtin__.pyi

@@ -538,7 +538,10 @@ 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 get(self, k: _KT, default: _VT = None) -> _VT: ...
+    @overload
+    def get(self, k: _KT) -> Optional[_VT]: ...
+    @overload
+    def get(self, k: _KT, default: _T) -> Union[_VT, _T]: ...
     def pop(self, k: _KT, default: _VT = ...) -> _VT: ...
     def popitem(self) -> Tuple[_KT, _VT]: ...
     def setdefault(self, k: _KT, default: _VT = ...) -> _VT: ...

stdlib/2/typing.pyi

@@ -174,15 +174,17 @@ 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_co]):
+class Mapping(Iterable[_KT], Container[_KT], Sized, Generic[_KT, _VT_co]):
     # TODO: We wish the key type could also be covariant, but that doesn't work,
     # see discussion in https: //github.com/python/typing/pull/273.
     @abstractmethod
     def __getitem__(self, k: _KT) -> _VT_co:
         ...
     # Mixin methods
-    def get(self, k: _KT, default: _VT_co = ...) -> _VT_co:  # type: ignore
-        ...
+    @overload  # type: ignore
+    def get(self, k: _KT) -> Optional[_VT_co]: ...
+    @overload  # type: ignore
+    def get(self, k: _KT, default: _T) -> Union[_VT_co, _T]: ...
     def keys(self) -> list[_KT]: ...
     def values(self) -> list[_VT_co]: ...
     def items(self) -> list[Tuple[_KT, _VT_co]]: ...

stdlib/3/builtins.pyi

@@ -556,7 +556,10 @@ 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 get(self, k: _KT, default: _VT = None) -> _VT: ...
+    @overload
+    def get(self, k: _KT) -> Optional[_VT]: ...
+    @overload
+    def get(self, k: _KT, default: _T) -> Union[_VT, _T]: ...
     def pop(self, k: _KT, default: _VT = None) -> _VT: ...
     def popitem(self) -> Tuple[_KT, _VT]: ...
     def setdefault(self, k: _KT, default: _VT = None) -> _VT: ...

stdlib/3/typing.pyi

@@ -250,8 +250,10 @@ class Mapping(Iterable[_KT], Container[_KT], Sized, Generic[_KT, _VT_co]):
     def __getitem__(self, k: _KT) -> _VT_co:
         ...
     # Mixin methods
-    def get(self, k: _KT, default: _VT_co = ...) -> _VT_co:  # type: ignore
-        ...
+    @overload  # type: ignore
+    def get(self, k: _KT) -> Optional[_VT_co]: ...
+    @overload  # type: ignore
+    def get(self, k: _KT, default: _T) -> Union[_VT_co, _T]: ...
     def items(self) -> AbstractSet[Tuple[_KT, _VT_co]]: ...
     def keys(self) -> AbstractSet[_KT]: ...
     def values(self) -> ValuesView[_VT_co]: ...