Deprecate positional arguments for read_fwf
- [x] xref #41485
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

Similar to read_csv this will simplify typing the return values
Added a test. 