Changing config items at runtime is broken

Changing config values at runtime should be possible, yet it's not working: https://docs.astropy.org/en/stable/config/index.html#changing-values-at-runtime

```
>>> from astroquery.simbad import conf
>>> conf.server='simbad.harvard.edu'
>>> from astroquery.simbad import Simbad
>>> Simbad.SIMBAD_URL
'http://simbad.u-strasbg.fr/simbad/sim-script'
```

Start with the runtime configuration guidance at the linked Astropy documentation and reproduce the example using astroquery.simbad.conf and Simbad.SIMBAD_URL. Trace why changing conf.server does not affect the URL, then verify that runtime configuration changes are reflected in the resulting Simbad setting.

Written by the indexing model from the issue text.