[Datetime-SIG] Proposal for a "time zone index" cache to be added to datetime

I have opened bpo-35723: "Add a time zone index cache" <https://bugs.python.org/issue35723> on the Python bug tracker, along with an associated proof of concept PR, PR #11529 <https://github.com/python/cpython/pull/11529>. I propose that some time zone offset information can be optionally cached on the datetime object itself, which could be a significant improvement to performance in tzinfo implementations using the standard interface. I think this would be a very useful change to the datetime object, but it is fairly significant, so I wanted to bring it to the attention of the datetime SIG. If it is deemed necessary, I can write a PEP for this. Best, Paul