title: pandas DataFrame.rename unexpected keyword argument "axis" when using mapper
body: <p>Following the <a href="https://pandas.pydata.org/pandas-docs/version/0.21/generated/pandas.DataFrame.rename.html" rel="noreferrer">pandas docs</a> I tried the following (verbatim out of the docs):</p>
  
  <pre><code>df = pd.DataFrame({"A": [1, 2, 3], "B": [4, 5, 6]})
  df.rename(str.lower, axis='columns')
  </code></pre>
  
  <p>yet I'm getting the error</p>
  
  <blockquote>
    <p>TypeError: rename() got an unexpected keyword argument "axis"</p>
  </blockquote>
  
  <p>I also tried</p>
  
  <pre><code>df.rename(mapper=str.lower, axis='columns')
  </code></pre>
  
  <p>but then I get:</p>
  
  <blockquote>
    <p>TypeError: rename() got an unexpected keyword argument "mapper"</p>
  </blockquote>
  
  <p>Am I looking at an old version of the docs?</p>