Use a compatibility function for method comparison

This is a rough attempt at building a compatibility function that
allows us to detect whether an object defines its own __reduce__ or
__reduce_ex__ on any interpreter. Some versions of PyPy 2.7 don't seem
to return True with "is" comparisons, and instead require ==.

I have arbitrarily chosen int as a builtin that derives from object and
doesn't define its own __reduce__. I think this is fairly safe to do in
2.7-only code, since it is stable now.

src/attr/_make.py

@@ -46,6 +46,19 @@
 _sentinel = object()
 
 
+if PY2 and object.__reduce__ is not int.__reduce__:
+    # In pypy2.7, you can't use `is` on methods, but we want to use whichever
+    # method tells us `object.__reduce__` equals `int.__reduce__`
+    def _method_eq(a, b):
+        return a == b
+
+
+else:
+
+    def _method_eq(a, b):
+        return a is b
+
+
 class _Nothing(object):
     """
     Sentinel class to indicate the lack of a value when ``None`` is ambiguous.
@@ -496,8 +509,8 @@ def __init__(
 
         if (
             cache_hash
-            and cls.__reduce__ is object.__reduce__
-            and cls.__reduce_ex__ is object.__reduce_ex__
+            and _method_eq(cls.__reduce__, object.__reduce__)
+            and _method_eq(cls.__reduce_ex__, object.__reduce_ex__)
         ):
             self._cls_dict["__reduce__"] = _cache_hash_reduce