Minor code style change

Author: Vasileios Karakasis

reframe/utility/sanity.py

@@ -538,8 +538,8 @@ def _callable_name(fn):
 
 def _extractiter_singletag(patt, filename, tag, conv, encoding):
     if isinstance(conv, collections.Iterable):
-        raise SanityError('multiple conversion functions given for the single '
-                          f'capturing group {tag!r}')
+        raise SanityError(f'multiple conversion functions given for the '
+                          f'single capturing group {tag!r}')
 
     for m in finditer(patt, filename, encoding):
         try: