Unify 2.7 and 3 stubs for functools.

stdlib/2.7/functools.pyi

@@ -1,6 +1,6 @@
 # Stubs for functools (Python 2.7)
-#
-# NOTE: This dynamically typed stub was automatically generated by stubgen.
+
+# NOTE: These are incomplete!
 
 from abc import ABCMeta, abstractmethod
 from typing import Any, Callable, Generic, Dict, Iterator, Optional, Sequence, Tuple, TypeVar
@@ -12,6 +12,10 @@ _T = TypeVar("_T")
 def reduce(function: Callable[[_T], _T],
            sequence: Iterator[_T], initial: Optional[_T] = ...) -> _T: ...
 
+# TODO implement as class; more precise typing
+# TODO cache_info and __wrapped__ attributes
+def lru_cache(maxsize: Optional[int] = ...) -> Callable[[Any], Any]: ...
+
 WRAPPER_ASSIGNMENTS = ... # type: Sequence[str]
 WRAPPER_UPDATES = ... # type: Sequence[str]