gh-126156: Improve performance of creating Morsel objects

Replaces the manually constructed loop with a call to `dict.update`

Author: bdraco

Lib/http/cookies.py

@@ -266,15 +266,16 @@ class Morsel(dict):
         "samesite" : "SameSite",
     }
 
+    _reserved_defaults = {key: "" for key in _reserved}
+
     _flags = {'secure', 'httponly'}
 
     def __init__(self):
         # Set defaults
         self._key = self._value = self._coded_value = None
 
         # Set default attributes
-        for key in self._reserved:
-            dict.__setitem__(self, key, "")
+        dict.update(self, self._reserved_defaults)
 
     @property
     def key(self):