Miscellaneous improvements for 2024-W16

[khaeru]

Some of these are motivated by usage in dsss:

• Add DataMessage.update() to update .observation_dimension automatically.
• Add StructureMessage.iter_objects() as a convenience combining .iter_collections() and .objects().
• Support URN construction for DataConsumerScheme, OrganizationScheme, ReportingTaxonomy, TransformationScheme, and VTLMappingScheme via sdmx.urn.make(…).

Others:

• ~Xfail tests of the ILO data source, which is sporadically offline.~ Adjust the URL for ILO per #177.
• Handle .json.Reader(…, structure=…); this led to TypeError in Client.get(…) with SDMX-JSON services.
• Split common .reader.xml code into its own submodule.
  • Rename DispatchingReader to more accurate XMLEventReader.

Housekeeping

• Update README so it renders correctly.
• Work around Python 3.8, 3.9 availability on macOS GitHub Actions runners.

PR checklist

• [x] Checks all ✅
• [x] Update documentation
• [x] Update doc/whatsnew.rst

[codecov[bot]]

Codecov Report

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

Project coverage is 97.87%. Comparing base (8d5e467) to head (4dc8043).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #175 +/- ## ========================================== - Coverage 98.86% 97.87% -1.00% ========================================== Files 94 95 +1 Lines 7431 7537 +106 ========================================== + Hits 7347 7377 +30 - Misses 84 160 +76 ``` | [Files](https://app.codecov.io/gh/khaeru/sdmx/pull/175?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto) | Coverage Δ | | |---|---|---| | [sdmx/message.py](https://app.codecov.io/gh/khaeru/sdmx/pull/175?src=pr&el=tree&filepath=sdmx%2Fmessage.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-c2RteC9tZXNzYWdlLnB5) | `100.00% <100.00%> (ø)` | | | [sdmx/model/common.py](https://app.codecov.io/gh/khaeru/sdmx/pull/175?src=pr&el=tree&filepath=sdmx%2Fmodel%2Fcommon.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-c2RteC9tb2RlbC9jb21tb24ucHk=) | `99.90% <100.00%> (ø)` | | | [sdmx/reader/base.py](https://app.codecov.io/gh/khaeru/sdmx/pull/175?src=pr&el=tree&filepath=sdmx%2Freader%2Fbase.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-c2RteC9yZWFkZXIvYmFzZS5weQ==) | `100.00% <100.00%> (ø)` | | | [sdmx/reader/json.py](https://app.codecov.io/gh/khaeru/sdmx/pull/175?src=pr&el=tree&filepath=sdmx%2Freader%2Fjson.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-c2RteC9yZWFkZXIvanNvbi5weQ==) | `100.00% <100.00%> (ø)` | | | [sdmx/reader/xml/common.py](https://app.codecov.io/gh/khaeru/sdmx/pull/175?src=pr&el=tree&filepath=sdmx%2Freader%2Fxml%2Fcommon.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-c2RteC9yZWFkZXIveG1sL2NvbW1vbi5weQ==) | `100.00% <100.00%> (ø)` | | | [sdmx/reader/xml/v21.py](https://app.codecov.io/gh/khaeru/sdmx/pull/175?src=pr&el=tree&filepath=sdmx%2Freader%2Fxml%2Fv21.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-c2RteC9yZWFkZXIveG1sL3YyMS5weQ==) | `99.15% <100.00%> (-0.11%)` | :arrow_down: | | [sdmx/reader/xml/v30.py](https://app.codecov.io/gh/khaeru/sdmx/pull/175?src=pr&el=tree&filepath=sdmx%2Freader%2Fxml%2Fv30.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-c2RteC9yZWFkZXIveG1sL3YzMC5weQ==) | `95.19% <100.00%> (ø)` | | | [sdmx/tests/reader/test\_base.py](https://app.codecov.io/gh/khaeru/sdmx/pull/175?src=pr&el=tree&filepath=sdmx%2Ftests%2Freader%2Ftest_base.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-c2RteC90ZXN0cy9yZWFkZXIvdGVzdF9iYXNlLnB5) | `100.00% <100.00%> (ø)` | | | [sdmx/tests/reader/test\_reader\_xml.py](https://app.codecov.io/gh/khaeru/sdmx/pull/175?src=pr&el=tree&filepath=sdmx%2Ftests%2Freader%2Ftest_reader_xml.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-c2RteC90ZXN0cy9yZWFkZXIvdGVzdF9yZWFkZXJfeG1sLnB5) | `100.00% <100.00%> (ø)` | | | [sdmx/tests/reader/test\_reader\_xml\_v21.py](https://app.codecov.io/gh/khaeru/sdmx/pull/175?src=pr&el=tree&filepath=sdmx%2Ftests%2Freader%2Ftest_reader_xml_v21.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-c2RteC90ZXN0cy9yZWFkZXIvdGVzdF9yZWFkZXJfeG1sX3YyMS5weQ==) | `100.00% <100.00%> (ø)` | | | ... and [5 more](https://app.codecov.io/gh/khaeru/sdmx/pull/175?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto) | | ... and [17 files with indirect coverage changes](https://app.codecov.io/gh/khaeru/sdmx/pull/175/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto)