API/BUG: Enforce "normalized" pytz timezones for DatetimeIndex
closes #3746
closes #18595
closes #13238
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Addressing 3 birds with 2 stones here. Using @pganssle suggested implementation for a tz property and directly raising an error per #3746 (could depreciate and error in a future version as well, open to feedback on the prefered path of API change)

Additionally, solves the issue of resampling a DataFrame/Series with a DatetimeIndex that retained a local timezone instead of the "LMT" version.  