Use nbclient instead in ixmp.testing.run_notebook()

[khaeru]

run_notebook() used the built-in Python subprocess module to run tutorial and test notebooks, write the results to a file, and then read the finished notebook. This process recently began to cause test & CI failures, e.g. in #370.

The Jupyter ecosystem now has a first-class package nbclient that executes notebooks programmatically; this PR:

• adjusts the function to use this package.
• replaces nbformat with nbclient as a requirement for the 'tests' extra set. (nbclient itself depends on nbformat.)
• moves and update codecov.yml; in the ci/ directory, it was not being detected by the Codecov CLI tool.

How to review

• Note that the CI checks all pass.

PR checklist

• ~Add or expand tests.~ N/A
• [x] Add, expand, or update documentation.
• ~Update release notes.~ N/A

[codecov[bot]]

Codecov Report

Merging #371 into master will decrease coverage by 0.0%. The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master    #371     +/-   ##
========================================
- Coverage    96.8%   96.8%   -0.1%     
========================================
  Files          46      46             
  Lines        5255    5253      -2     
========================================
- Hits         5088    5086      -2     
  Misses        167     167             

Impacted Files	Coverage Δ
ixmp/testing.py	78.2% <100.0%> (-0.2%)	:arrow_down:

[khaeru]

It was necessary to work around jupyter/nbclient#85; this was done according to https://github.com/jupyter/nbconvert/issues/1372#issuecomment-691596163. The workaround should be removed once that issue is fixed in a version of nbclient ≥ 0.5.1.