[Code Cleanup] Fix a broad except call

It is enough to catch `UnknownTimeZoneError` here.

calsocial/forms.py

@@ -59,7 +59,7 @@ class TimezoneField(SelectField):
 
         try:
             self.data = pytz.timezone(value)
-        except Exception as exc:
+        except pytz.exceptions.UnknownTimeZoneError:
             raise ValueError(f'Unknown time zone {value}')
 
     def iter_choices(self):