support python3.9+ xml parsing

[calgray]

xmi parser used by tango-yaml xmi <file> currently no longer works on python 3.9+ due to removal of getchildren

Definition of Done Checklist

• Code meets our python style guidelines?
• Unit tested (coded, passed, included)?
• Requested at least 2 reviewers?
• Commented code, particularly in hard-to-understand areas?
• Made corresponding changes to the documentation (e.g. Python documentation, System Engineering Documentation, version description updates, README file, etc)?

JIRA: JIRA_ISSUE_NUMBER