MRG: Add defusedxml dependency to "full" variant

[hoechenberger]

Addresses the problem we saw in #1217

Merge checklist

Maintainer, please confirm the following before merging. If applicable:

• [x] All comments are resolved
• [x] This is not your own PR
• [x] All CIs are happy
• [x] PR title starts with [MRG]
• [x] whats_new.rst is updated
• [x] New contributors have been added to CITATION.cff
• [x] PR description includes phrase "closes <#issue-number>"

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (af3fefb) 97.61% compared to head (0d97f5d) 97.55%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1218 +/- ## ========================================== - Coverage 97.61% 97.55% -0.06% ========================================== Files 40 40 Lines 8685 8685 ========================================== - Hits 8478 8473 -5 - Misses 207 212 +5 ```

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