Implement/ensure compat with SDMX 3.0

[khaeru]

Draft standards for SDMX 3.0 were published on 2021-06-05: summary of major changes — full details.
The standards will be finalized at the 8th SDMX Global Conference, which ends 2021-09-30.

This issue is to consider whether and how to implement SDMX 3.0 in sdmx1, and maybe later to track other issues and PRs.

Possible steps:

• Rename sdmx.model to something like sdmx.model.v2_1 or …v21.
• Add a parallel module named sdmx.model.v3_0 or …v30, and possibly a common sdmx.model.common for classes that are not changed.
• Consider a similar split for sdmx.reader.xml, sdmx.reader.json, sdmx.writer.xml, sdmx.writer.json, and sdmx.client.
• Capture version support in sources.json etc.
• Implement and release in stages:
  • Create a new release with 2.1 code clearly marked and raising NotImplementedError for most behaviour for 3.0.
  • Mark direct import of e.g. sdmx.model deprecated, recommending sdmx.model.v21.
  • Expand 3.0 support incrementally.
  • Eventually remove deprecated direct imports etc.

Considerations:

• Order of hierarchy. Is it better to have sdmx.model.v21 or sdmx.v21.model?
• Common vs. separate code.
  • Common code could be shorter, but involve a lot of branching that would increase complexity and could impact performance.
  • Separate code might involve duplication (not DRY, but judiciously WET?). It may be necessary or even a cleaner solution for strict enforcement of the Information Model/typing, where those details change between 2.1 and 3.0.
• Timing. Many web services still exist which support only SDMX 2.0, when 2.1 was published in 2011 (!). It is not clear if or when particular agencies plan to offer SDMX 3.0 web services; and if or when they would remove support for SDMX 2.1.