Update implementation of TimedCache object

There were some odd decisions made about the implementation of some of
the required methods for MuttableMapping in the TimedCache object. This
cleans those up and makes the implementation, ever so slightly, easier
to read.

See also #3885

requests/structures.py

@@ -148,10 +148,10 @@ class TimedCache(collections.MutableMapping):
             (self.maxlen, len(self._dict), self.expiration_secs)
 
     def __iter__(self):
-        return map(lambda kv: (kv[0], kv[1][1]), self._dict.items()).__iter__()
+        return ((key, value[1]) for key, value in self._dict.items())
 
     def __delitem__(self, item):
-        return self._dict.__delitem__(item)
+        del self._dict[item]
 
     def __getitem__(self, key):
         """
@@ -166,7 +166,7 @@ class TimedCache(collections.MutableMapping):
 
         if now - occurred > self.expiration_secs:
             del self._dict[key]
-            raise KeyError
+            raise KeyError(key)
         else:
             return value
 
@@ -183,7 +183,7 @@ class TimedCache(collections.MutableMapping):
         while len(self._dict) >= self.maxlen:
             self._dict.popitem(last=False)
 
-        return self._dict.__setitem__(key, (now, value))
+        self._dict[key] = (now, value)
 
     def __len__(self):
         """:return: the length of the cache"""