Fix Codecov status checks and Python 3.6 build

[khaeru]

Addresses two CI issues:

• Codecov is not correctly attaching status checks to message_ix PRs after GitHub Actions workflow runs.
  • This is in contrast to ixmp, which works.
  • The Codecov action used in the workflow seems to have some limitations, see e.g. The yml optional input doesn't seem to be working codecov/codecov-action#51 (comment). This might be related because, unlike ixmp, the message_ix GHA configuration checks the repo out into a subdirectory.
  • Replaced with step that manually downloads and runs the codecov script.
  • See Add "working-directory:" input codecov/codecov-action#110—once this is accepted/merged, we can return to using the action.
• The same issue as Run CI checks on Python 3.6 ixmp#359: the checks for Python 3.6 were actually being run with Python 3.8. This allowed Set duration_period in add_horizon()  #286 to pass and be merged, even though it contained syntax not compatible with Python 3.6.

How to review

Confirm that Codecov checks are shown on this PR.

PR checklist

• Add or expand tests. N/A, CI changes only
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

Merging #381 into master will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #381      +/-   ##
==========================================
+ Coverage   90.46%   90.55%   +0.08%     
==========================================
  Files          31       32       +1     
  Lines        2780     2806      +26     
==========================================
+ Hits         2515     2541      +26     
  Misses        265      265              

Impacted Files	Coverage Δ
message_ix/tests/test_core.py	100.00% <ø> (ø)
message_ix/core.py	95.20% <100.00%> (ø)
message_ix/message_ix/tests/test_tutorials.py
message_ix/message_ix/testing/nightly.py
message_ix/message_ix/cli.py
message_ix/message_ix/tests/test_feature_addon.py
...ix/message_ix/tests/test_feature_bound_emission.py
message_ix/message_ix/reporting/__init__.py
message_ix/message_ix/reporting/pyam.py
message_ix/message_ix/tests/tools/test_add_year.py
... and 55 more

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