Fix #15862 remove dirs in keep list when walk path

@s0undt3ch You can't assign a new list to dirs if you want change dirs in `os.walk`.

But you can change it `in place`

    In [1]: lst1 = [1,2,3,4,5,6,7,8,9]
    In [5]: id(lst1)
    Out[5]: 25487048
    In [6]: lst1 = [l for l in lst1 if l != 5]
    In [7]: lst1
    Out[7]: [1, 2, 3, 4, 6, 7, 8, 9]
    In [8]: id(lst1)
    Out[8]: 25504296

-------------------

    In [9]: lst2 = [1,2,3,4,5,6,7,8,9]
    In [10]: id(lst2)
    Out[10]: 25486112
    In [11]: lst2[:] = [l for l in lst2 if l != 5]
    In [12]: id(lst2)
    Out[12]: 25486112