BUG: GH35865 - int cast to str with colon setitem
- [x] closes #35865 
- [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
- [ ] whatsnew entry: not sure if it's needed but happy to provide one given some guidelines

Don't think we need a whatsnew. Thx for adding the tests thanks @arredond 