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

Author: sigmavirus24

requests/structures.py

@@ -148,10 +148,10 @@ def __repr__(self):
             (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 @@ def __getitem__(self, key):
 
         if now - occurred > self.expiration_secs:
             del self._dict[key]
-            raise KeyError
+            raise KeyError(key)
         else:
             return value
 
@@ -183,7 +183,7 @@ def __setitem__(self, key, value):
         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"""