Avoid using fstrings until python35 support is dropped

The PR for that (#722) is
yet to have landed

Author: cderici

juju/model.py

@@ -2607,7 +2607,7 @@ def _raise_for_status(entities, status):
 
         if wait_for_exact_units is not None:
             assert type(wait_for_exact_units) == int and wait_for_exact_units >= 0, \
-                f'Invalid value for wait_for_exact_units {wait_for_exact_units}'
+                'Invalid value for wait_for_exact_units : %s' % wait_for_exact_units
 
         while True:
             busy = []