Import Literal type from typing_extensions if python version<3.8 (#33)

Author: hukkinj1

immutables/_map.pyi

@@ -3,7 +3,6 @@ from typing import Generic
 from typing import Hashable
 from typing import Iterable
 from typing import Iterator
-from typing import Literal
 from typing import Mapping
 from typing import MutableMapping
 from typing import NoReturn
@@ -78,7 +77,7 @@ class MapMutation(MutableMapping[K, V]):
     def __init__(self, count: int, root: BitmapNode) -> None: ...
     def set(self, key: K, val: V) -> None: ...
     def __enter__(self: S) -> S: ...
-    def __exit__(self, *exc: Any) -> Literal[False]: ...
+    def __exit__(self, *exc: Any): ...
     def __iter__(self) -> NoReturn: ...
     def __delitem__(self, key: K) -> None: ...
     def __setitem__(self, key: K, val: V) -> None: ...