Delete f-string in codebase to preserve python 3.5 support.
Delete f-string in codebase to preserve python 3.5 support.

We currently support python 3.5+, but f-strings come in 3.6.

Fixes #7124
jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab