Changed public interface to mathtools.factors(). CHANGE.

Function previously included 1.

Function no longer includes 1.

OLD:

    >>> for n in range(10, 20):
    ...   print(n, mathtools.factors(n))
    ...
    10 [1, 2, 5]
    11 [1, 11]
    12 [1, 2, 2, 3]
    13 [1, 13]
    14 [1, 2, 7]
    15 [1, 3, 5]
    16 [1, 2, 2, 2, 2]
    17 [1, 17]
    18 [1, 2, 3, 3]
    19 [1, 19]

NEW:

    >>> for n in range(10, 20):
    ...   print(n, mathtools.factors(n))
    ...
    10 [2, 5]
    11 [11]
    12 [2, 2, 3]
    13 [13]
    14 [2, 7]
    15 [3, 5]
    16 [2, 2, 2, 2]
    17 [17]
    18 [2, 3, 3]
    19 [19]

CHANGE: should be included in the 2.16 change log.