Fix missing conversion of label values to string

[City-busz]

This fixes the following error when opening Stream Information:

Traceback (most recent call last):
  File "/usr/lib/hypnotix/hypnotix.py", line 1390, in open_info
    v = Gtk.Label(label=prop_v, margin_top=12, margin_bottom=12)
  File "/usr/lib/python3.14/site-packages/gi/overrides/__init__.py", line 357, in new_init
    return super_init_func(self, **new_kwargs)
TypeError: Must be string, not int

Tested with Python 3.14.7 and PyGObject 3.56.3.