old_contents: """
  A set of abstract widgets which require a subclass to be useful.
  """
  
  from wtforms.widgets.core import Input, HTMLString
  from abc import ABCMeta
  from ..common import CustomWidgetMixin
  import wtforms.widgets.core as wt_core
  
  class BootstrapPlainCheckboxRadio(wt_core.CheckboxInput, CustomWidgetMixin):
      """
      Abstract widget for a Bootstrap Checkbox or Radio implementation.
      """
      __metaclass__ = ABCMeta
      def __call__(self, field, **kwargs):
          label = getattr(field, 'label', None)
          if label in kwargs:
              label = kwargs.pop('label').strip()
  
          html = """
          <div class="{input_type}">
              <label>{rendered_field}{label}</label>
          </div>
          """.format(
              label=label,
              input_type=self.input_type,
              rendered_field=super(BootstrapPlainCheckboxRadio, self).__call__(field, **kwargs)
          )
          return HTMLString(html)
new_contents: """
  A set of abstract widgets which require a subclass to be useful.
  """
  
  from wtforms.widgets.core import Input, HTMLString
  from abc import ABCMeta
  from ..common import CustomWidgetMixin
  import wtforms.widgets.core as wt_core
  
  class BootstrapPlainCheckboxRadio(wt_core.CheckboxInput, CustomWidgetMixin):
      """
      Abstract widget for a Bootstrap Checkbox or Radio implementation.
      """
      __metaclass__ = ABCMeta
      def __call__(self, field, **kwargs):
          label = getattr(field, 'label', None)
          if label in kwargs:
              label = kwargs.pop('label').strip()
  
          html = u"""
          <div class="{input_type}">
              <label>{rendered_field}{label}</label>
          </div>
          """.format(
              label=label,
              input_type=self.input_type,
              rendered_field=super(BootstrapPlainCheckboxRadio, self).__call__(field, **kwargs)
          )
          return HTMLString(html)
  
subject: make Checkbox Unicode clean