Adjust typing for mypy 0.960 & other minor fixes

[khaeru]

• Closes Adjust for mypy 0.960 #449
• In .reporting, uses the improved functionality of Computer.require_compat() from genno 1.11 to ensure ixmp.reporting.computations are available for use.
• Add and document the Model.enforce() method for adjusting scenario contents to be consistent with model representations; introduced downstream in Resolve the issue of zero capacity factor message_ix#561, also used in Regional emission pools message_ix#514.

How to review

• Read the diff and note that the CI checks all pass.

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 #450 (de3df62) into main (8f8c74b) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #450   +/-   ##
=====================================
  Coverage   98.6%   98.6%           
=====================================
  Files         42      42           
  Lines       4441    4446    +5     
=====================================
+ Hits        4379    4384    +5     
  Misses        62      62           

Impacted Files	Coverage Δ
ixmp/core/scenario.py	97.9% <100.0%> (ø)
ixmp/model/base.py	100.0% <100.0%> (ø)
ixmp/reporting/reporter.py	100.0% <100.0%> (ø)