['c3', 'd4', 'c1', 'd3', 'b1', 'a4', 'b3', 'c4', 'a3', 'b2', 'a1', 'c2',
    'd1', 'a2', 'b4', 'd2']
['d4', 'b3', 'c1', 'a4', 'b2', 'c4', 'd3', 'a1', 'c3', 'a2', 'b4', 'd2',
    'a3', 'b1', 'c2', 'd1']
shuffling a list with restrictions in Python