Fix abstract classes for Python 3 (#2239)

* add metaclass=ABCMeta to some classes

* mark some more classes as explicitly abstract

* make some more classes concrete

stdlib/3/collections/__init__.pyi

@@ -286,9 +286,12 @@ class Counter(Dict[_T, int], Generic[_T]):
 
 _OrderedDictT = TypeVar('_OrderedDictT', bound=OrderedDict)
 
-class _OrderedDictKeysView(KeysView[_KT], Reversible[_KT]): ...
-class _OrderedDictItemsView(ItemsView[_KT, _VT], Reversible[Tuple[_KT, _VT]]): ...
-class _OrderedDictValuesView(ValuesView[_VT], Reversible[_VT]): ...
+class _OrderedDictKeysView(KeysView[_KT], Reversible[_KT]):
+    def __reversed__(self) -> Iterator[_KT]: ...
+class _OrderedDictItemsView(ItemsView[_KT, _VT], Reversible[Tuple[_KT, _VT]]):
+    def __reversed__(self) -> Iterator[Tuple[_KT, _VT]]: ...
+class _OrderedDictValuesView(ValuesView[_VT], Reversible[_VT]):
+    def __reversed__(self) -> Iterator[_VT]: ...
 
 class OrderedDict(Dict[_KT, _VT], Reversible[_KT], Generic[_KT, _VT]):
     def popitem(self, last: bool = ...) -> Tuple[_KT, _VT]: ...

stdlib/3/importlib/abc.pyi

@@ -47,7 +47,7 @@ class ExecutionLoader(InspectLoader):
     def get_filename(self, fullname: str) -> _Path: ...
     def get_code(self, fullname: str) -> Optional[types.CodeType]: ...
 
-class SourceLoader(ResourceLoader, ExecutionLoader):
+class SourceLoader(ResourceLoader, ExecutionLoader, metaclass=ABCMeta):
     def path_mtime(self, path: _Path) -> Union[int, float]: ...
     def set_data(self, path: _Path, data: bytes) -> None: ...
     def get_source(self, fullname: str) -> Optional[str]: ...
@@ -82,7 +82,7 @@ if sys.version_info >= (3, 3):
                 target: Optional[types.ModuleType] = ...
             ) -> Optional[ModuleSpec]: ...
 
-    class FileLoader(ResourceLoader, ExecutionLoader):
+    class FileLoader(ResourceLoader, ExecutionLoader, metaclass=ABCMeta):
         name = ...  # type: str
         path = ...  # type: _Path
         def __init__(self, fullname: str, path: _Path) -> None: ...

stdlib/3/os/__init__.pyi

@@ -139,6 +139,11 @@ X_OK: int
 
 class _Environ(MutableMapping[AnyStr, AnyStr], Generic[AnyStr]):
     def copy(self) -> Dict[AnyStr, AnyStr]: ...
+    def __delitem__(self, key: AnyStr) -> None: ...
+    def __getitem__(self, key: AnyStr) -> AnyStr: ...
+    def __setitem__(self, key: AnyStr, value: AnyStr) -> None: ...
+    def __iter__(self) -> Iterator[AnyStr]: ...
+    def __len__(self) -> int: ...
 
 environ: _Environ[str]
 if sys.version_info >= (3, 2):
@@ -456,6 +461,7 @@ else:
     def rmdir(path: _PathType) -> None: ...
 if sys.version_info >= (3, 7):
     class _ScandirIterator(Iterator[DirEntry[AnyStr]], ContextManager[_ScandirIterator[AnyStr]]):
+        def __next__(self) -> DirEntry[AnyStr]: ...
         def close(self) -> None: ...
     @overload
     def scandir() -> _ScandirIterator[str]: ...
@@ -465,6 +471,7 @@ if sys.version_info >= (3, 7):
     def scandir(path: Union[AnyStr, PathLike[AnyStr]]) -> _ScandirIterator[AnyStr]: ...
 elif sys.version_info >= (3, 6):
     class _ScandirIterator(Iterator[DirEntry[AnyStr]], ContextManager[_ScandirIterator[AnyStr]]):
+        def __next__(self) -> DirEntry[AnyStr]: ...
         def close(self) -> None: ...
     @overload
     def scandir() -> _ScandirIterator[str]: ...

stdlib/3/typing.pyi

@@ -178,7 +178,7 @@ class Coroutine(Awaitable[_V_co], Generic[_T_co, _T_contra, _V_co]):
 # NOTE: This type does not exist in typing.py or PEP 484.
 # The parameters corrrespond to Generator, but the 4th is the original type.
 class AwaitableGenerator(Awaitable[_V_co], Generator[_T_co, _T_contra, _V_co],
-                         Generic[_T_co, _T_contra, _V_co, _S]):
+                         Generic[_T_co, _T_contra, _V_co, _S], metaclass=ABCMeta):
     pass
 
 @runtime
@@ -465,7 +465,7 @@ class TextIO(IO[str]):
     @abstractmethod
     def __enter__(self) -> TextIO: ...
 
-class ByteString(Sequence[int]): ...
+class ByteString(Sequence[int], metaclass=ABCMeta): ...
 
 class Match(Generic[AnyStr]):
     pos = 0