BUG: Series(dt64, dtype="Sparse[object]")
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

As a side effect of fixing astype_nansafe, we can rip some code out of Block._astype.