Revert "don't require metadata"
Revert "don't require metadata"

This reverts commit 90e3898aedb47e5bff87b71ed1de35e573117985.

mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext