Add support for validating SDMX-ML v2.1 messages

[goatsweater]

Required: write a single sentence that describes the changes made by this PR.

Add support for validating SDMX-ML messages via sdmx.validate_xml() (Closes #51)

PR checklist

• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[khaeru]

@goatsweater many thanks for the contribution here!

The test failures are due to added specimens for which the support is in #152. So for next steps:

• I will complete and merge that PR.
• I will ask you to rebase this branch on main (or I can do that if you don't feel comfortable).
• Once the checks pass, you can complete the PR description and checklist above.
• You can request a review from me.

…and then we will go from there.

I will also investigate the settings for this repo so I don't need to manually approve the CI runs on every push you (or other contributors) make to your branch.

[goatsweater]

@khaeru I saw that you had made some changes since my fork and merged those into this branch. I think that may have messed up the ability to rebase. I'm happy to close this and start over with a proper rebase if you'd like.

[khaeru]

I think that may have messed up the ability to rebase. I'm happy to close this and start over with a proper rebase if you'd like.

If you don't mind, that would be appreciated. Please let me know if you get stuck and I can try to create a new branch by cherry-picking some commits from this one.

In the meantime I'll look at the CLA setup, as promised.

[goatsweater]

Closing in favor of #154