chore: remove warnings from defusedxml package

[diegovr]

Description

Issue: openedx/public-engineering#146

• Refactor the deprecated defusedxml package which will no longer be supported.
  • The current python version used(python3.8) supports the xml.etree.ElementTree package which replace the old package (defusedxml).
• Fixed the Deprecation Warnings by making this change.

Supporting information

The best references to why it was deprecated I could find are here:

• tostring(RestrictedElement('X')) produces funky output tiran/defusedxml#25
• import defusedxml.lxml.html tiran/defusedxml#31

Testing instructions

To check and find from where I get the same warning, I run the unit tests inside the lms container (by running make dev.shell.lms) and I could found that the tests which use the deprecated package are from openedx/core/lib/tests/ path so I used pytest to confirm that.

Then after the changes I re-run the tests and the warning no longer appeared.

Other information

• This change can also be the first step to deprecate/remove the defusedxml package since is used only in this section.