[FIX] tests: remove f-string in classCleanUp backport

This commit is a fixup to the earlier commits that backport CPython's
3.8 classCleanups, due to the fact that the official supported python
version for Odoo V12 is CPython 3.5, where f-strings are not available.

This one slipped through due to the fact that v12 runbot does not
actually run under Python 3.5 but 3.6+

closes odoo/odoo#73074

Signed-off-by: Adrian Torres (adt) <adt@odoo.com>