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.

[glopezdiest]

Nothing to add to the discussion, approved