use a Protocol for str.format_map() (#4122)

Fixes #3824

stdlib/2and3/builtins.pyi

@@ -407,6 +407,9 @@ else:
 
     _str_base = basestring
 
+class _FormatMapMapping(Protocol):
+    def __getitem__(self, __key: str) -> Any: ...
+
 class str(Sequence[str], _str_base):
     if sys.version_info >= (3,):
         @overload
@@ -433,7 +436,7 @@ class str(Sequence[str], _str_base):
     def find(self, sub: Text, __start: Optional[int] = ..., __end: Optional[int] = ...) -> int: ...
     def format(self, *args: object, **kwargs: object) -> str: ...
     if sys.version_info >= (3,):
-        def format_map(self, map: Mapping[str, Any]) -> str: ...
+        def format_map(self, map: _FormatMapMapping) -> str: ...
     def index(self, sub: Text, __start: Optional[int] = ..., __end: Optional[int] = ...) -> int: ...
     def isalnum(self) -> bool: ...
     def isalpha(self) -> bool: ...