Implement IXMP4Backend

[khaeru]

This PR is intended to aid @glatterf42 in developing model data features in ixmp4. It is not a complete plan or solution for migrating ixmp-based code, workflows, and research to ixmp4; such a plan would involve many additional elements that are not in scope for this PR.

The PR:

• Adds a IXMP4Backend class that allows message_ix (thus all existing code based on ixmp and messageix) to talk to ixmp4's API. This functions as a **[shim](https://en.wikipedia.org/wiki/Shim(computing))** between the established and new/developing APIs.
• Makes minimal changes to allow ixmp.Platform backed by IXMP4Backend to be created and used.
• Adjusts or parametrizes tests of the ixmp Platform, TimeSeries, and Scenario classes to run against both JDBCBackend and IXMP4Backend, ensuring the latter provides the same behaviour expected of the former per the test suite.
  • This is a precondition for migrating other code (for instance, in message-ix-models or message_data) to, first, use ixmp4 via an IXMP4Backend, and eventually, ixmp4 directly.
• May serve as a basis for a parallel PR in message_ix.

Notes

• ixmp4 is not published on PyPI for Python 3.8 and 3.9 and can't be installed in those CI jobs.
  • [ ] Separate ixmp4 from the tests dependency, and don't install it on those Pythons.
  • [ ] Adjust parametrized tests to not import ixmp4 or run against IXMP4Backend with those Pythons.
  • [ ] Document that IXMP4Backend is only available for Python >= 3.10.

How to review

TBD. This branch/PR may not necessarily be merged itself, but may serve as the basis for another PR that adds the IXMP4Backend.

PR checklist

• [ ] Continuous integration checks all ✅
• [ ] Add or expand tests; coverage checks both ✅
• [ ] Add, expand, or update documentation.
• [ ] Update release notes.

[khaeru]

The Python 3.12 job fails with different exceptions than 3.10 and 3.11, for instance here:

ixmp/backend/ixmp4.py:15: in __init__
    import ixmp4
[...]
/opt/hostedtoolcache/Python/3.12.1/x64/lib/python3.12/site-packages/sqlalchemy/orm/decl_api.py:303: in __get__
    reg[self] = obj = self.fget(cls)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

cls = <class 'ixmp4.data.db.unit.model.Unit'>

    @declared_attr.directive
    def columns(cls):
>       return cls.__table__.columns
E       AttributeError: type object 'Unit' has no attribute '__table__'. Did you mean: '__le__'?

/opt/hostedtoolcache/Python/3.12.1/x64/lib/python3.12/site-packages/ixmp4/data/db/base.py:76: AttributeError

I guess this means ixmp4 does not support Python 3.12, although it is installable.