Fix dispid caching in dynamic.CDispatch.__getattr__

Property 'dispid's only found by __AttrToID__() / GetIDsOfNames()
were not cached in propMap and caused time consuming search upon
every read.

Author: kxrob

com/win32com/client/dynamic.py

@@ -599,6 +599,7 @@ def __call__(self):
                             retEntry = self._olerepr_.propMapGet.get(attr)
                     if retEntry is None:
                         retEntry = build.MapEntry(self.__AttrToID__(attr), (attr,))
+                        self._olerepr_.propMap[attr] = retEntry
                 except pythoncom.ole_error:
                     pass  # No prop by that name - retEntry remains None.