Add defusedxml to extra requirements for xml readers

[mraspaud]

This PR adds defusedxml as an extra requirement for some of the xml-based readers in Satpy.

This issue was reported by @lobsiger in the pytroll mailing list https://groups.google.com/g/pytroll/c/cXhNKmMleGg and the solution was suggest by @djhoese in this message https://groups.google.com/g/pytroll/c/cXhNKmMleGg/m/yCXcjpFHAQAJ

[codecov[bot]]

Codecov Report

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

Comparison is base (07a448e) 95.40% compared to head (2c64ff8) 95.40%. Report is 10 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2715 +/- ## ======================================= Coverage 95.40% 95.40% ======================================= Files 371 371 Lines 52749 52825 +76 ======================================= + Hits 50323 50399 +76 Misses 2426 2426 ``` | [Flag](https://app.codecov.io/gh/pytroll/satpy/pull/2715/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [behaviourtests](https://app.codecov.io/gh/pytroll/satpy/pull/2715/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `4.15% <ø> (-0.01%)` | :arrow_down: | | [unittests](https://app.codecov.io/gh/pytroll/satpy/pull/2715/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `96.02% <ø> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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[coveralls]

Pull Request Test Coverage Report for Build 7472531915

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.968%

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Totals
Change from base Build 7472442128:	0.0%
Covered Lines:	50525
Relevant Lines:	52648

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💛 - Coveralls

[mraspaud]

The changes here seem to have made their way into main, closing.