Replace lxml with standard library xml

[vkbo]

When Python 3.7 is dropped some time after mid 2023, see PEP 537, the lxml package can potentially be replaced by the standard library xml module. It would get rid of one more dependency.

There are some known issues with the implementation of the standard library xml compared to lxml:

• Prior to 3.8, the attribute order is not preserved by the write method. This is not an issue for novelWriter at runtime, but it breaks all tests that compare written XML files.
• Prior to 3.9, there is no function for indenting the XML file. The function itself is trivial, and can be copied from the standard library source, so this is not really a blocker.
• In general, the implementation of namespaces is a lot clunkier in the standard library xml module. It is much easier to handle in lxml.

Edit: Python 3.7 end of life set to 2023-06-27. See devguide.python.org.