Fix wrong docstring for unzip()

[lumbric]

The current version of unzip() works only if the inner sequence is
finite, but in Python 3 zip() can be used. See also comment in #239.

[eriknw]

Aha, right you are. Thanks, @lumbric! I performed the following experiment to convince myself:

In [1]: from toolz.sandbox import unzip                                                                      

In [2]: import itertools                                                                                     

In [3]: seq = ((i, i+10, i+20) for i in itertools.count())                                                   

In [4]: next(seq)                                                                                            
Out[4]: (0, 10, 20)

In [5]: next(seq)                                                                                            
Out[5]: (1, 11, 21)

In [6]: next(seq)                                                                                            
Out[6]: (2, 12, 22)

In [7]: a, b, c = unzip(seq)                                                                                 

In [8]: next(a)                                                                                              
Out[8]: 3

In [9]: next(a)                                                                                              
Out[9]: 4

In [10]: next(b)                                                                                             
Out[10]: 13

In [11]: next(b)                                                                                             
Out[11]: 14

In [12]: seq2 = zip(a, b, c)                                                                                 

In [13]: next(seq2)                                                                                          
Out[13]: (5, 15, 23)

In [14]: next(seq2)                                                                                          
Out[14]: (6, 16, 24)

In [15]: next(seq2)                                                                                          
Out[15]: (7, 17, 25)

I also tried unzip([itertools.count(i) for i in range(5)]), which, as now expected, exploded memory usage.