Prefer f-strings

This gets rid of some errors from the pyupgrade pre-commit hook.

Author: jsf9k

src/cyhy_runner/cyhy_runner.py

@@ -186,7 +186,7 @@ def main():
 
     group = args["--group"]
     if group:
-        print('Setting effective group to "{}".'.format(group), file=sys.stderr)
+        print(f'Setting effective group to "{group}".', file=sys.stderr)
 
         new_gid = grp.getgrnam(group).gr_gid
         os.setegid(new_gid)
@@ -196,7 +196,7 @@ def main():
     working_dir = os.path.join(os.getcwd(), args["<working-dir>"])
     if not os.path.exists(working_dir):
         print(
-            'Working directory "{}" does not exist.'.format(working_dir),
+            f'Working directory "{working_dir}" does not exist.',
             end="",
             file=sys.stderr,
         )