The edx-sandbox (secure/codejailed execution context
for edx-platform) still runs in Python 3.5, so certain
modules in common/lib cannot be py-upgraded to use
Python 3.8 syntax.

Specifically, this commit added an f-string to
common/lib/sandbox-packages/verifiers/draganddrop.py,
which broke scoring of deprecated drag-and-drop problems
in codejail.

TNL-8322

This reverts commit 8c06b68bbe4e502fe0bb81eb7e3eb1481d864dfc