Fixed value validation in DataLabel.overflow. Fixes #198.

Author: hcpchris

highcharts_core/utility_classes/data_labels.py

@@ -561,8 +561,8 @@ def overflow(self, value):
             self._overflow = None
         else:
             value = value.lower()
-            if value not in ['justify', 'none']:
-                raise errors.HighchartsValueError(f'overflow accepts "justify" or "none".'
+            if value not in ['justify', 'allow', 'none']:
+                raise errors.HighchartsValueError(f'overflow accepts "justify", "allow", or "none".'
                                                   f' Was: {value}')
             self._overflow = value