Python 3.9+ support for route_parser; deprecated xml code.

[Daraan]

This PR adds the already merged one-line fix from carla-simulator/scenario_runner#1053

Replace xml.etree.ElementTree.Element.getchildren method which is deprecated from python 3.2 and removed in python 3.9 with a suggested replacement https://docs.python.org/3.8/library/xml.etree.elementtree.html#xml.etree.ElementTree.Element.getchildren

- for elem in scenario.getchildren():
+ for elem in list(scenario):

Tested with Python 3.7 and 3.10+; from the deprecation notice should work with 3.2+

---

This change is 

[Daraan]

@PabloVD can you please merge this PR so a manual fix is not longer needed when someone wants to use a higher python version. Thanks a lot.

[PabloVD]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Daraan)

[PabloVD]

Merged, thanks!