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

third_party/2and3/google/protobuf/internal/containers.pyi

@@ -3,17 +3,21 @@ from google.protobuf.internal.message_listener import MessageListener
 from google.protobuf.message import Message
 from typing import (
     MutableSequence, Sequence, TypeVar, Generic, Any, Iterator, Iterable,
-    Union, Optional, Callable
+    Union, Optional, Callable, overload, List
 )
 
 _T = TypeVar('_T')
-class BaseContainer(Generic[_T], MutableSequence[_T]):
+class BaseContainer(Sequence[_T]):
     def __init__(self, message_listener: MessageListener) -> None: ...
     def __len__(self) -> int: ...
     def __ne__(self, other: object) -> bool: ...
     def __hash__(self) -> int: ...
     def __repr__(self) -> str: ...
     def sort(self, *, key: Optional[Callable[[_T], Any]] = ..., reverse: bool = ...) -> None: ...
+    @overload
+    def __getitem__(self, key: int) -> _T: ...
+    @overload
+    def __getitem__(self, key: slice) -> List[_T]: ...
 
 class RepeatedScalarFieldContainer(Generic[_T], BaseContainer[_T]):
     def __init__(self, message_listener: MessageListener, message_descriptor: Descriptor) -> None: ...

third_party/2and3/requests/packages/urllib3/response.pyi

@@ -1,4 +1,5 @@
-from typing import Any, IO
+from typing import Any
+import io
 from . import _collections
 from . import exceptions
 from .connection import HTTPException as HTTPException, BaseSSLError as BaseSSLError
@@ -22,7 +23,7 @@ class GzipDecoder:
     def __getattr__(self, name): ...
     def decompress(self, data): ...
 
-class HTTPResponse(IO[Any]):
+class HTTPResponse(io.IOBase):
     CONTENT_DECODERS = ...  # type: Any
     REDIRECT_STATUSES = ...  # type: Any
     headers = ...  # type: Any

third_party/3/pkg_resources.pyi

@@ -5,6 +5,7 @@ from typing import (
     List, Union,
     TypeVar, overload,
 )
+from abc import ABCMeta
 import importlib.abc
 import sys
 import types
@@ -248,9 +249,9 @@ class ExtractionError(Exception):
 
 
 if sys.version_info >= (3, 3):
-    class _Importer(importlib.abc.MetaPathFinder, importlib.abc.InspectLoader): ...
+    class _Importer(importlib.abc.MetaPathFinder, importlib.abc.InspectLoader, metaclass=ABCMeta): ...
 else:
-    class _Importer(importlib.abc.InspectLoader): ...
+    class _Importer(importlib.abc.InspectLoader, metaclass=ABCMeta): ...
 
 def register_finder(importer_type: type,
                     distribution_finder: _DistFinderType) -> None: ...