Handle validFrom, validTo, and annotations on ItemSchemes

[khaeru]

Closes #142.

PR checklist

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

[codecov]

Codecov Report

Merging #143 (4cdd7d2) into main (7363a51) will decrease coverage by 2.41%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #143      +/-   ##
==========================================
- Coverage   98.27%   95.87%   -2.41%     
==========================================
  Files          87       87              
  Lines        6437     6445       +8     
==========================================
- Hits         6326     6179     -147     
- Misses        111      266     +155     

Files	Coverage Δ
sdmx/reader/xml/v21.py	97.69% <100.00%> (-0.13%)	⬇️
sdmx/testing/__init__.py	99.28% <ø> (ø)
sdmx/tests/reader/test_reader_xml_v21.py	100.00% <100.00%> (ø)

... and 17 files with indirect coverage changes

[khaeru]

@goatsweater can you please let me know if you are able to test this branch? You could install with something like:

pip install --upgrade "sdmx1 @ git+https://github.com/khaeru/sdmx.git@issue/142"

…and then use it against the "internal services" you mentioned in #142.

If that would take a long time or is otherwise not possible, that's fine; I'll just merge and make a patch release with the fix.

[goatsweater]

I installed the branch and re-ran the services. I am now appropriately seeing annotations and valid to/from dates.