Include Python version 3.10 and remove 3.6 in CI

[LauWien]

This PR includes Python version 3.10 and removes Python version 3.6 in CI.
Closes #532

How to review

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

PR checklist

• Continuous integration checks all ✅
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

Merging #566 (c5ab939) into main (1df3619) will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            main    #566     +/-   ##
=======================================
- Coverage   93.8%   93.7%   -0.1%     
=======================================
  Files         40      40             
  Lines       3057    3057             
=======================================
- Hits        2870    2867      -3     
- Misses       187     190      +3     

Impacted Files	Coverage Δ
message_ix/tests/test_tutorials.py	88.4% <0.0%> (-11.6%)	⬇️

[khaeru]

Sorry for not advertising more loudly, but I made these changes incidentally in the branch for #557. I was expecting to merge that quickly, but it is stalled on #559.

[LauWien]

Sorry for not advertising more loudly, but I made these changes incidentally in the branch for #557. I was expecting to merge that quickly, but it is stalled on #559.

Oh shoot, sorry I didn't check deeply!
I started #565 to fix #559, by first forcing to use the new pandas release 1.4.1, since the test_add_year.py was running through successful locally with that version. The tests were also successful on GHA with pandas==1.4.1, but then I saw with that new errors occur, see here. So I thought, I should maybe first check the offending lines here f7c1374 which led to the same failing tests here like described in #565, so I reverted 6c3ef88 and started this new PR here to first tackle the failing tests with pandas==1.4.1 on Python version 3.6 and after the failing tests in ubuntu-latest.
So maybe we don't need to adapt the offending lines since with the latest pandas version the test test_add_year.py, which failed previously, were successful, but remove Python version 3.6 and tackle the failing ubuntu-latest tests. What do you think?

Regarding the failing ubuntu-test e.g. the following error:

File /opt/hostedtoolcache/Python/3.8.12/x64/lib/python3.8/site-packages/message_ix/models.py:359, in GAMSModel.run(self, scenario)
E               340 """Execute the model.
E               341 
E               342 GAMSModel creates a file named ``cplex.opt`` in the model directory
E              (...)
E               348    each process may produced unexpected results.
E               349 """
E               350 # If two runs are kicked off simulatenously with the same
E               351 # self.model_dir, then they will try to write the same optfile, and may
E               352 # write different contents.
E              (...)
E               357 
E               358 # Write CPLEX options into an options file
E           --> 359 optfile = self.model_dir / "cplex.opt"
E               360 lines = ("{} = {}".format(*kv) for kv in self.cplex_opts.items())
E               361 optfile.write_text("\n".join(lines))
E           
E           TypeError: unsupported operand type(s) for /: 'str' and 'str'
E           TypeError: unsupported operand type(s) for /: 'str' and 'str'

It seems like the whitespace in line 359 optfile = self.model_dir / "cplex.opt" is ignored, which leads to a string division.

[khaeru]

This TypeError is the very thing that #557 is meant to address.

[LauWien]

This TypeError is the very thing that #557 is meant to address.

🤦‍♀️ sorry..
So then I guess I can close #566 and #565, since with pandas==1.4.1 the test_add_year.py is successful.