title: Pandas: drop certain values from a column with a string title
body: <p>Say I have the following dataframe <code>df</code>:</p>
  
  <pre><code>        First C        Second C       Third C
  0       0.104000       0.864000       -999
  1       0.060337       0.812470       -999
  2       0.065797       0.819570       0.802607
  3       0.064715       0.817212       0.801755
  </code></pre>
  
  <p>I want to drop the first two lines because column <code>Third C</code> shows two weird values. </p>
  
  <pre><code>df = df.drop(df[df.('Third C') == -999].index)
  </code></pre>
  
  <p>This throws:</p>
  
  <pre><code>       df = df.drop(df[df.('Third C') == -999].index)
                            ^
  SyntaxError: invalid syntax
  </code></pre>
  
  <p>And the same thing happens if I use <code>df.['Third C']</code> with square brackets. How can I perform this operation without having to rename my column?</p>