title: Convert flat list to dictionary with keys at regular intervals
body: <p>I have a list containing a string and lists. I should show you;</p>
  
  <pre><code>list_x = ['a', ['j', '1', 'x'], ['k', '2', 'y'], ['a', '3', 'hj'],
            'd', ['b', '4', 'df'], ['c', '5', 'er'], ['d', '6', 'ty'],
            'g', ['e', '7', 'hj'], ['f', '8', 'bv'], ['g', '9', 'sad'],
            'j', ['h', '10', 'kj'], ['i', '11', 'nbv'], ['c', '12', 'uy'],
            'n', ['d', '13', 'ipoas'], ['e', '14', 'fg'], ['f', '15', 'as'], 
            'r', ['g', '16', 'dsad'], ['h', '17', 'fdgdfg'], ['i', '18', 'retrt'],
            'u', ['j', '19', 'qwe'], ['k', '20', 'ytgf'], ['n', '21', 'asmz']]
  </code></pre>
  
  <p>And I want a dict from this list like this;</p>
  
  <pre><code>dict_x = {'a': [['j', '1', 'x'], ['k', '2', 'y'], ['a', '3', 'hj']],
            'd': [['b', '4', 'df'], ['c', '5', 'er'], ['d', '6', 'ty']],
            'g': [['e', '7', 'hj'], ['f', '8', 'bv'], ['g', '9', 'sad']],
            'j': [['h', '10', 'kj'], ['i', '11', 'nbv'], ['c', '12', 'uy']],
            'n': [['d', '13', 'ipoas'], ['e', '14', 'fg'], ['f', '15', 'as']], 
            'r': [['g', '16', 'dsad'], ['h', '17', 'fdgdfg'], ['i', '18', 'retrt']],
            'u': [['j', '19', 'qwe'], ['k', '20', 'ytgf'], ['n', '21', 'asmz']]}
  </code></pre>