Fix list indexing negative values

[ltorres6]

minor change to allow for negative indexing (e.g. last element of list "-1"). Tests passing locally.

For example:

from dotty_dict import Dotty

i_dict = Dotty(
    {"some": {"nested": {"list": {"here": {"now": [0, 1, 2, 3]}}}}}
)

key = "some.nested.list.here.now.-1"
value = i_dict[key]
print(f"dict value: {value}")
print(f"key in i_dict is {key in i_dict}")
print(f"value type: {type(value)}")

should return

dict value: 3
key in i_dict is True
value type: <class 'int'>