title: Cannot import statsmodels.formula.api
body: <p>Whenever i am trying to import statsmodels.formula.api as smf i get the following error.</p>
  
  <pre><code>import statsmodels.formula.api as smf
  
  Traceback (most recent call last):
  
  File "&lt;ipython-input-257-268d740a5877&gt;", line 1, in &lt;module&gt;
     import statsmodels.formula.api as smf
  
  File "C:\Users\ldresl\Anaconda3\lib\site- 
      packages\statsmodels\formula\__init__.py", line 1, in &lt;module&gt;
    from statsmodels import PytestTester
  
   ImportError: cannot import name 'PytestTester'
  </code></pre>