BUG: Construct Timestamp with tz correctly near DST border
closes #19970
closes #20854
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

So I _think_ the reason we need to convert a `Timestamp` to `datetime` branch is because of how `Timestamp` behave with `normalize`  explained by this https://github.com/pandas-dev/pandas/pull/18618#issuecomment-352271183