Adjust CI for missing codecov

[khaeru]

…by disabling the jobs that rely on the currently installable ixmp[tests] <= 3.6.0. If codecov is not re-established on PyPI (cf. iiasa/ixmp#477), these could possibly be re-enabled by adjusting to depend on ixmp only (not [tests]) and directly specifying here the additional dependencies (except codecov) that would have been brought in by ixmp[tests].

Along with other housekeeping:

• Move packaging to pyproject.toml, in parallel with iiasa/ixmp#476, iiasa/message_ix#703.
• Simplify "pytest" CI workflow.
• Reuse "publish" CI workflow from iiasa/actions.

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.~
• ~Update doc/whatsnew.~

[codecov[bot]]

Codecov Report

Merging #96 (bcff1cc) into main (66bcdac) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #96   +/-   ##
=======================================
  Coverage   67.09%   67.09%           
=======================================
  Files          58       58           
  Lines        4009     4009           
=======================================
  Hits         2690     2690           
  Misses       1319     1319