Change wait_for_transaction_receipt timeout type sig from float to Optional[float]

[ariel-simnegar]

Right now w3.eth.wait_for_transaction_receipt works just fine with timeout=None, because timeout is passed to asyncio.wait_for which accepts an Optional[float] timeout.

I think explicitly supporting timeout=None is useful for when wait_for_transaction_receipt is a subroutine in higher-level code which wants to specify an optional timeout and trivially delegate that to wait_for_transaction_receipt.