[REDO] CLN: core/dtypes/cast.py::maybe_downcast_to_dtype 
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

xref https://github.com/pandas-dev/pandas/pull/37050#issuecomment-708487786

The combination of #37050 and #37024 generated a `mypy` complaint. This PR is resubmission of #37050 with the correct type hint.

cc @simonjayhawkins
thanks @arw2019 