meson: do not use f-strings

Our travis CI still uses python3.5. I'm making this into a separate
commit to make it easy to revert later.

src/test/generate-sym-test.py

@@ -13,9 +13,9 @@ for line in open(sys.argv[1]):
     if match:
         s = match.group(1)
         if s == 'sd_bus_object_vtable_format':
-            print(f'    &{s},')
+            print('    &{},'.format(s))
         else:
-            print(f'    {s},')
+            print('    {},'.format(s))
 
 print('''};