Replace deprecated python 3.8 code and add missing __init__.py files

[ksuszka]

Description

• Add missing srunner/osc2/__init__.py file and fix a misspell in srunner/osc2/ast_manager/__init__.py file name.
• 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

Where has this been tested?

• Platform(s): Ubuntu 22.04
• Python version(s): 3.10.12
• Unreal Engine version(s): 4.26
• CARLA version: 0.9.15

Possible Drawbacks

None.

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This change is 

[ksuszka]

@ttgamage I'm not sure what merge policy is here in place. When I clicked notification email about your approval, there was a button here to rebase this change on top of the latest changes so I clicked it, but it seems that you need to approve it again.

[Daraan]

This PR definitely will be needed for later python version and for those it can be approved.

Some things on my mind:

• Are we sure that it also works with the current lower versions for backward compatibility, i.e. do these changes work with the current requirements.txt?
• The requirements.txt might need to be updated accordingly, or a second introduced for the later python versions.

[Daraan]

I tested with the requirements file and Python3.7

list(element) also works in Python2 (which is not really compatible with the requirements given)

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ksuszka)

[glopezdiest]

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ksuszka)

[glopezdiest]

Nothing to add to the discussion, approved