Tuple is returned as list when interpolating

[Ching-Hwa Yu]

from config import config

def test_tuple_interpolate():
    DICT = {"a1": (1, 2)}
    cfg = config(DICT, interpolate=True)
    assert cfg["a1"] == (1, 2)

Results in:

>       assert cfg["a1"] == (1, 2)
E       assert [1, 2] == (1, 2)
E         Full diff:
E         - (1, 2)
E         + [1, 2]

[Ching-Hwa Yu]

I've narrowed down the location of the bug and will submit a PR soon.