BUG: Breaking change to offsets.pyx - MonthOffset
Add ability to use class `MonthBegin` as a period.
It would address #38914 and #38859.

- [ ] closes #38914 #38859
- [x] tests added / passed
- [ ] 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
