Use reusable workflows for "lint", "pytest"

[khaeru]

This PR replaces the "lint" and "pytest" CI workflows defined within ixmp with others reused from iiasa/actions#8. See “Reusing workflows” in the GitHub docs.

This reduces the maintenance burden, i.e. minor updates to the workflow can be made in iiasa/actions and are then automatically effective for all other repositories which reuse it. Previously these changes had to be made up to 4 times—for ixmp, message_ix, message-ix-models, and message_data.

How to review

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

PR checklist

• [x] Continuous integration checks all ✅
• [x] ~Add or expand tests;~ coverage checks both ✅
• ~Add, expand, or update documentation.~ N/A; CI changes only
• ~Update release notes.~

[codecov[bot]]

Codecov Report

Merging #466 (46d93f6) into main (61e863c) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #466   +/-   ##
=====================================
  Coverage   98.5%   98.5%           
=====================================
  Files         42      42           
  Lines       4468    4472    +4     
=====================================
+ Hits        4404    4408    +4     
  Misses        64      64           

Impacted Files	Coverage Δ
ixmp/backend/base.py	100.0% <100.0%> (ø)
ixmp/core/platform.py	98.9% <100.0%> (ø)
ixmp/tests/backend/test_base.py	98.9% <100.0%> (+<0.1%)	:arrow_up: