BUG: fixing tseries plot cursor display, resolves #5453

Fixes #5453

I am not sure how to test a mouse-over, and the original commit 922c6102eebb7347cea587fffc4795a3ca3e73b2 introducing this functionality did not have tests either, which is sadly probably how this got broken. @willfurnass do you happen to have any ideas for how to test this functionality, maybe from when you first looked at this?

The new code doesn't display timezone information like the old code once did (before it was broken) as Periods don't seem to have timezone info, and all ```tseries/plotting.py``` plots are Period-based. One could perhaps do some minor acrobatics to convert the tz-naive start time of the Period into a tz-aware Timestamp, I can research that if people feel strongly about it.