Update & improve reporting

[khaeru]

• Address errors and deprecation warnings appearing in scheduled pytest jobs after the release of genno 1.18.
• Add the model_periods() operator and "y::model" and "y0" reporting keys; these are copied from iiasa/message-ix-models#116, and were originally in (private) message_data.
• Refresh reporting docs.
• Drop support for Python 3.7, which reached EOL on 2023-06-27 https://peps.python.org/pep-0537/#lifespan

How to review

• Read the diff.
• Note that the CI checks all pass.

PR checklist

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

[codecov[bot]]

Codecov Report

Merging #738 (25826cf) into main (bf44691) will decrease coverage by 0.2%. The diff coverage is 95.0%.

@@           Coverage Diff           @@
##            main    #738     +/-   ##
=======================================
- Coverage   94.5%   94.3%   -0.2%     
=======================================
  Files         43      43             
  Lines       3463    3475     +12     
=======================================
+ Hits        3273    3280      +7     
- Misses       190     195      +5     

Files Changed	Coverage Δ
message_ix/tests/test_tutorials.py	97.5% <ø> (ø)
message_ix/reporting/computations.py	97.1% <66.6%> (-2.9%)	:arrow_down:
message_ix/reporting/__init__.py	100.0% <100.0%> (ø)
message_ix/tests/test_reporting.py	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

[khaeru]

@glatterf42 I let the checks finish on the third-last commit to confirm they pass, then pushed two more commits that are changes to docs only.