jinja2 made a backwards incompatible change in its 3.1 release that broke a few packages around. nbconvert was one of them, they released a couple of days ago a new version with a fix for jinja2. However nbsite is pinning nbconvert to an older version <6, that is very unlikely to be patched by nbconvert. While updating nbsite to support nbconvert 6 should be a goal at some point (or using another client), for now nbsite will just pin jinja2 < 3.1.
jinja2 made a backwards incompatible change in its 3.1 release that broke a few packages around. nbconvert was one of them, they released a couple of days ago a new version with a fix for jinja2. However nbsite is pinning nbconvert to an older version <6, that is very unlikely to be patched by nbconvert. While updating nbsite to support nbconvert 6 should be a goal at some point (or using another client), for now nbsite will just pin jinja2 < 3.1.