Introduce Python3 compatibility in example

[amarco90]

The example in examples/foo_complex.py does not work in Python3 due to the division operator.

Description

I fixed the error by importing the division operator from __future__ and using the integer division operator (//).

Motivation and Context

When running the example using Python3 I was getting the following error:

  File "/Users/amarco/Desktop/luigi/examples/foo_complex.py", line 64, in requires
    for i in range(valor / max_depth):
TypeError: 'float' object cannot be interpreted as an integer

Have you tested this? If so, how?

I ran the example task and it completed successfully:

===== Luigi Execution Summary =====
Scheduled 51 tasks of which:
* 51 ran successfully:
    - 50 examples.Bar(num=0,1,10,11,12,13,14,15,16,17,18,...)
    - 1 examples.Foo()

This progress looks :) because there were no failed tasks or missing external dependencies
===== Luigi Execution Summary =====

[mention-bot]

@amarco90, thanks for your PR! By analyzing the history of the files in this pull request, we identified @hadesbox and @Tarrasch to be potential reviewers.

[Tarrasch]

Thanks! :)