Type fixes for versioning.py

[gycsaba96]

The changes are as follows:

• use the _Element/_ElementTree class instead of the Element/ElementTree factory for typing,
• expect None values returned by lxml,
• other minor fixes.

Two unresolved lines still reference the core/xml.py file removed in e43eee0. This doesn't cause runtime errors because the code in versioning.py is currently unreachable (see PR #1133; this is possibly the reason behind issue #1051).