CI: filter json DeprecationWarning
- [x] closes #39854
- [ ] 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

We're already using the same filter in two other places in this file.
@jreback gentle ping gets the CI back to green thanks @jbrockmendel  @meeseeksdev backport 1.2.x