Adjust for strict type hinting in ixmp

[khaeru]

iiasa/ixmp#581 by @glatterf42 expanded type hints to the entire code base.

This PR makes adjusts message_ix to address mypy errors arising from those changes, and take advantages of typing-related utilities added.

This PR does not apply the same mypy settings for message_ix as in ixmp; that may be done in a later PR.

How to review

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. N/A
• Update release notes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.1%. Comparing base (9fce44b) to head (ae9f89d).
Report is 13 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #963   +/-   ##
=====================================
  Coverage   96.0%   96.1%           
=====================================
  Files         53      53           
  Lines       5026    5030    +4     
=====================================
+ Hits        4829    4834    +5     
+ Misses       197     196    -1     

Files with missing lines	Coverage Δ
message_ix/cli.py	96.9% <ø> (+1.2%)	⬆️
message_ix/core.py	97.4% <100.0%> (-0.1%)	⬇️
message_ix/models.py	99.2% <100.0%> (+<0.1%)	⬆️
message_ix/tests/test_report.py	100.0% <100.0%> (ø)
message_ix/util/__init__.py	98.6% <100.0%> (-0.1%)	⬇️
message_ix/util/ixmp4.py	100.0% <100.0%> (ø)
message_ix/util/scenario_setup.py	95.4% <100.0%> (ø)