title: Count specific field in dataframe groupby
body: <p>I'm new to Python and trying to get my head around how to manipulate Pandas dataframes. I'm using the winemag-data-130k-v2.csv dataset. 
  The fields of interest are 'country','province','winery'variety'.</p>
  
  <p>The first thing I'd like to do is determine the number of wineries per province.
  I can get as far as 
  <code>reviews_df.groupby(['country','province']).size()</code></p>
  
  <p>But this gives me the number of rows. (So, 3 if a winery produces 3 varieties).
  But I want something like a <code>count(distinct winery)</code> in SQL.
  Suggestions?</p>