Simplify & improve configuration

[khaeru]

• Closes Remove Python 3.6 support #434, Exclude Python version 3.6 in CI #428.
• Since we thus now require Python 3.7, use dataclasses from the standard library to streamline and simplify the Config class.
• Fix configuration bugs: Setting the message model dir does not work  #415.
• Add configuration enhancements:
  • Set Platform jvmargs via config.json #408
  • Support Read GAMS CPLEX solver options for MESSAGE from user config message_ix#557.

Also:

• Update actions and steps in CI workflows to latest versions of those actions, Python, etc.

How to review

• Note the CI checks all pass.
• Optionally, play around with Setting the message model dir does not work  #415 on one's local machine and check that the setting is respected.

PR checklist

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

[codecov]

Codecov Report

Merging #435 (e9e9283) into main (08533cb) will increase coverage by 0.0%.
The diff coverage is 99.3%.

@@          Coverage Diff          @@
##            main    #435   +/-   ##
=====================================
  Coverage   98.5%   98.5%           
=====================================
  Files         41      41           
  Lines       4321    4380   +59     
=====================================
+ Hits        4259    4318   +59     
  Misses        62      62           

Impacted Files	Coverage Δ
ixmp/tests/backend/test_base.py	98.9% <ø> (ø)
ixmp/tests/backend/test_jdbc.py	100.0% <ø> (ø)
ixmp/_config.py	96.8% <98.8%> (+1.0%)	⬆️
ixmp/backend/jdbc.py	95.2% <100.0%> (+<0.1%)	⬆️
ixmp/testing/__init__.py	95.8% <100.0%> (-0.1%)	⬇️
ixmp/testing/data.py	97.2% <100.0%> (ø)
ixmp/testing/resource.py	83.0% <100.0%> (ø)
ixmp/tests/test_cli.py	100.0% <100.0%> (ø)
ixmp/tests/test_compat.py	100.0% <100.0%> (ø)
ixmp/tests/test_config.py	100.0% <100.0%> (ø)
... and 2 more

[khaeru]

@LauWien the review request is mainly FYI per the workflow and packaging updates, which we could replicate in our other repos. No need to do a deep dive on the code changes; only skim.