TYP: Fix chainmap typing for mypy 0.740+
This will otherwise raise a failure during a `mypy` update. Also increase type precision of this file to 100%.

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

cc @simonjayhawkins 