title: Pandas: convert unicode strings to string
body: <p>I have data frame and there are 2 columns with unicode value. I need to convert it to string. I try <code>df.domain.astype(str)</code> but it return unicode strings.
  How can I do that?
  Data looks like (I need to convert either columns)</p>
  
  <pre><code>domain       search_term
  vk.com           None
  facebook.com     None
  yandex.ru        снять квартиру
  locals.ru        None
  yandex.ru        снять квартиру без посредников в Москве
  avito.ru         None
  </code></pre>