Add explicit default= for schema_version and describedBy fields

[dbirman]

Without this PR you get pylance warnings that there is no argument set for describedBy and schema_version when you try to create any of the core models. By setting the default the warnings go away. No tests change, but not knowing pydantic deeply I don't know if this has other consequences (I don't think it does).

Could also be that my pylance settings are junk?

closes #1076 closes #1077

Adding an image to show what the warning looks like: