Allow non-ascii characters in sites.json (when enough astropy user base supports it)

#38 fixed an issue described in #36 where astropy doesn't correctly parse the ``sites.json`` file when it has non-ascii characters. While #38 fixes it, it does so by removing some characters that are proper names that really ought to be unicode. astropy/astropy#7082 fixes the underlying problem by making everything unicode-compliant in astropy. So this issue is about esentially reverting #38, and, more broadly, allowing UTF-8 in the ``site.json`` file.

We do *not* want to do this yet, however, as it will instantly break all released astropys (at the time of this writing). So instead, we are thinking to follow the suggestion of https://github.com/astropy/astropy-data/issues/36#issuecomment-358695793 - wait a while until we think most installed astropy have the fix (could be a long time...), and then bring in the non-ascii characters again.