Fix mypy error messages in pandas/test/tseries/offsets/test_offsets.py
Activate and fix mypy for test offsets 

- xref #28926
- [X] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

lgtm @simonjayhawkins  Great thanks @samuelsinayoko 