weakproxy fixes (#2450)

* The callback argument to ref() and proxy() is optional, per documentation
* proxy() returns a ProxyType or CallableProxyType

Closes #1627

stdlib/2and3/_weakref.pyi

@@ -1,6 +1,7 @@
 import sys
-from typing import Any, Callable, Generic, Optional, TypeVar
+from typing import Any, Callable, Generic, Optional, TypeVar, overload
 
+_C = TypeVar('_C', bound=Callable[..., Any])
 _T = TypeVar('_T')
 
 class CallableProxyType(object):  # "weakcallableproxy"
@@ -12,8 +13,7 @@ class ProxyType(object):  # "weakproxy"
 class ReferenceType(Generic[_T]):
     if sys.version_info >= (3, 4):
         __callback__: Callable[[ReferenceType[_T]], Any]
-    def __init__(self, o: _T, callback: Callable[[ReferenceType[_T]],
-                                                 Any] = ...) -> None: ...
+    def __init__(self, o: _T, callback: Optional[Callable[[ReferenceType[_T]], Any]] = ...) -> None: ...
     def __call__(self) -> Optional[_T]: ...
     def __hash__(self) -> int: ...
 
@@ -21,4 +21,8 @@ ref = ReferenceType
 
 def getweakrefcount(object: Any) -> int: ...
 def getweakrefs(object: Any) -> int: ...
-def proxy(object: _T, callback: Callable[[_T], Any] = ...) -> ref[_T]: ...
+@overload
+def proxy(object: _C, callback: Optional[Callable[[_C], Any]] = ...) -> CallableProxyType: ...
+# Return CallableProxyType if object is callable, ProxyType otherwise
+@overload
+def proxy(object: _T, callback: Optional[Callable[[_T], Any]] = ...) -> Any: ...