Address flaky TestILO.test_gh_96()

[khaeru]

TestILO.test_gh_96() is flaky, as the server often returns 503 Server Error, leading to failure of 1 or more jobs in the daily scheduled CI run. To avoid this, use a stored response via sdmx-test-data instead of making an actual/network query. The test no longer hinges on the exact performance of a 'live' server.

Also:

• Change the default base URL for the ILO source to use https:// instead of http://.

PR checklist

• Checks all ✅
• Update documentation N/A, test changes only.
• Update doc/whatsnew.rst

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.10%. Comparing base (deca38f) to head (47970fd).
⚠️ Report is 99 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
- Coverage   98.92%   98.10%   -0.83%     
==========================================
  Files         103      103              
  Lines        8687     8686       -1     
==========================================
- Hits         8594     8521      -73     
- Misses         93      165      +72     

Files with missing lines	Coverage Δ
sdmx/tests/test_sources.py	92.99% <100.00%> (-3.28%)	⬇️

... and 20 files with indirect coverage changes

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