Mark 3 tests as flaky

[khaeru]

Closes #238.

PR checklist

• Checks all ✅
• Update documentation N/A, tests/CI only.
• Update doc/whatsnew.rst

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.25%. Comparing base (59f003d) to head (a2db448).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #245      +/-   ##
==========================================
- Coverage   98.89%   98.25%   -0.65%     
==========================================
  Files         114      114              
  Lines        9326     9329       +3     
==========================================
- Hits         9223     9166      -57     
- Misses        103      163      +60     

Files with missing lines	Coverage Δ
sdmx/tests/format/test_xml.py	100.00% <100.00%> (ø)

... and 17 files with indirect coverage changes

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