BUG: DTI/TDI.insert doing invalid casting
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

> does this closes any issues?

Not AFAICT looks good, can you merge master and ping on green. rebased+green