Improve Python 2 stub for abc; _weakrefset stub.

builtins/2.7/_weakrefset.pyi

@@ -0,0 +1,5 @@
+from typing import Iterator, Any
+
+class WeakSet:
+    def __iter__(self) -> Iterator[Any]: ...
+    def add(self, *args, **kwargs) -> Any: ...

stdlib/2.7/abc.pyi

@@ -1,5 +1,38 @@
-# Stubs for abc.
+from typing import Any, Dict, Set, Union, Tuple
+import _weakrefset
 
-# Thesee definitions have special processing in type checker.
-class ABCMeta: ...
-abstractmethod = object()
+# mypy has special processing for ABCMeta and abstractmethod.
+
+WeakSet = ...  # type: _weakrefset.WeakSet
+_InstanceType = ...  # type: type
+types = ...  # type: module
+
+def abstractmethod(funcobj) -> Any: ...
+
+class ABCMeta(type):
+    # TODO: FrozenSet
+    __abstractmethods__ = ...  # type: Set[Any]
+    __doc__ = ...  # type: str
+    _abc_cache = ...  # type: _weakrefset.WeakSet
+    _abc_invalidation_counter = ...  # type: int
+    _abc_negative_cache = ...  # type: _weakrefset.WeakSet
+    _abc_negative_cache_version = ...  # type: int
+    _abc_registry = ...  # type: _weakrefset.WeakSet
+    def __init__(self, name, bases, namespace: Dict[Any, Any]) -> None: ...
+    def __instancecheck__(cls: "ABCMeta", instance) -> Any: ...
+    def __subclasscheck__(cls: "ABCMeta", subclass) -> Any: ...
+    def _dump_registry(cls: "ABCMeta", *args, **kwargs) -> None: ...
+    # TODO: subclass: Union["ABCMeta", type, Tuple[type, ...]]
+    def register(cls: "ABCMeta", subclass: Any) -> None: ...
+
+class _C:
+    pass
+
+# TODO: The real abc.abstractproperty inherits from "property".
+class abstractproperty(object):
+    __doc__ = ...  # type: str
+    __isabstractmethod__ = ...  # type: bool
+    doc = ...  # type: Any
+    fdel = ...  # type: Any
+    fget = ...  # type: Any
+    fset = ...  # type: Any