This function name was changed in Django 1.10 and removed in 2.0.

def _format_value(self, value):
        """This function name was changed in Django 1.10 and removed in 2.0."""
        # Use renamed format_name() for Django versions >= 1.10.
        if hasattr(self, 'format_value'):
            return super(DateTimePicker, self).format_value(value)
        # Use old _format_name() for Django versions < 1.10.
        else:
            return super(DateTimePicker, self)._format_value(value)