stdlib: fix many attributes which are read-only at runtime but read-write in the stub (#7395)

stdlib/http/cookies.pyi

@@ -21,9 +21,12 @@ def _unquote(str: str) -> str: ...
 class CookieError(Exception): ...
 
 class Morsel(dict[str, Any], Generic[_T]):
-    value: str
-    coded_value: _T
-    key: str
+    @property
+    def value(self) -> str: ...
+    @property
+    def coded_value(self) -> _T: ...
+    @property
+    def key(self) -> str: ...
     def __init__(self) -> None: ...
     if sys.version_info >= (3, 7):
         def set(self, key: str, val: str, coded_val: _T) -> None: ...