BUG: lib.infer_dtype with mixed-freq Periods
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Also adds a convert_period option to lib.maybe_infer_objects
