Handle int values in TimeSeries.timeseries(year=[…])

[khaeru]

Closes #440.

How to review

• Read the diff, especially the added tests.
• Note that the CI checks all pass, including these tests.

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 #469 (09be3d7) into main (fefa80d) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #469   +/-   ##
=====================================
  Coverage   98.5%   98.5%           
=====================================
  Files         42      42           
  Lines       4481    4486    +5     
=====================================
+ Hits        4417    4422    +5     
  Misses        64      64           

Impacted Files	Coverage Δ
ixmp/core/timeseries.py	98.7% <ø> (ø)
ixmp/tests/test_utils.py	100.0% <ø> (ø)
ixmp/tests/core/test_timeseries.py	100.0% <100.0%> (ø)

[khaeru]

@Jihoon requesting your review here as you were the one who filed #440.