title: The activate() method doesn't return the activated sheet in xlwings on assignment
body: <p>I'm writing a program to convert Excel (.xlsx) files to json files using <code>Python 3.5</code> and <code>xlwings 0.10.0</code>. Everything is working great except I ran into a hiccup when trying to add an option for iterating over the sheets.</p>
  
  <pre><code>import xlwings as xw
  
  wb = xw.Book('myfile.xlsx')
  for s in wb.sheets:
      sheet = s.activate()
      print(sheet.range(1, 1))
  </code></pre>
  
  <p>This simplified code snippet sums up my problem.
  In <code>xlwings</code> <a href="http://docs.xlwings.org/en/stable/api.html" rel="nofollow noreferrer">documentation</a> it is stated that the <code>activate()</code> method activates the sheet <em>and</em> returns it. Following that logic I would expect <code>sheet = s.activate()</code> to assign the current iterated and activated sheet to the variable <code>sheet</code>. Yet it throws an <code>AttributeError: 'NoneType' object has no attribute 'range'</code> error when executing the last line (<code>sheet</code> returns None instead of the activated sheet).</p>
  
  <p>What worked for me was separating the sheet activation and assignment:</p>
  
  <pre><code>import xlwings as xw
  wb = xw.Book('myfile.xlsx')
  
  for s in wb.sheets:
      s.activate()
      sheet = s
      print(sheet.range(1, 1))
  </code></pre>
  
  <p>I'd like to find out why the first snippet doesn't work and the second does.</p>
  
  <p>If I have overlooked something basic I should know, please feel free to point that out since I'm quite new to development.</p>