Backport PR #2458: fix: Use str object for jupyterlite-sphinx jupyterlite_dir config
* Backport PR https://github.com/scikit-hep/pyhf/pull/2458
* To avoid warning
WARNING: The config value `jupyterlite_dir` has type `PosixPath`, defaults to `str`.
use a str for the jupyterlite-sphinx v0.13.0+ jupyterlite_dir config
option.
Backport PR #2458: fix: Use str object for jupyterlite-sphinx jupyterlite_dir config