title: WTForms "too many values to unpack" with SelectField
body: <p>I'm using WTForms and I'm trying to display a <code>SelectField</code>, but I get the following error:</p>
  
  <pre><code>&gt;&gt;&gt; form.status()
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "C:\Python26\Lib\site-packages\wtforms\fields.py", line 136, in __call__
      return self.widget(self, **kwargs)
    File "C:\Python26\Lib\site-packages\wtforms\widgets.py", line 237, in __call__
      for val, label, selected in field.iter_choices():
    File "C:\Python26\Lib\site-packages\wtforms\fields.py", line 390, in iter_choices
      for value, label in self.choices:
  ValueError: too many values to unpack
  </code></pre>
  
  <p>Here's my form:</p>
  
  <pre><code>class TestForm(Form):
      status = SelectField(u'Status', choices=Test.statuses())
  </code></pre>
  
  <p>The <code>Test.statuses</code> static method returns a list of strings.  What am I doing wrong?</p>