Fix watchman tests

[christophe-david]

This PR puts back on track our tests with last versions of FAST-OAD dependencies:

The modifications that impact users are:

• the minimum required version of OpenMDAO is now 3.18
• a bug has been fixed, that could cause a crash when getting the list of variables from an OpenMDAO problem if N2 reports were activated.

On dev side, the version of Poetry for GitHub workflows is now controlled by the repository variable POETRY_VERSION (currently set to 1.4.1). Poetry lock file has been set accordingly.

[codecov[bot]]

Codecov Report

Merging #483 (b35c1b2) into master (2b40991) will increase coverage by 0.28%. The diff coverage is 100.00%.

:exclamation: Current head b35c1b2 differs from pull request most recent head 0e57386. Consider uploading reports for the commit 0e57386 to get more accurate results

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
+ Coverage   83.42%   83.70%   +0.28%     
==========================================
  Files         126      126              
  Lines        5797     5776      -21     
  Branches      869      865       -4     
==========================================
- Hits         4836     4835       -1     
+ Misses        805      787      -18     
+ Partials      156      154       -2     

Impacted Files	Coverage Δ
src/fastoad/openmdao/_utils.py	100.00% <100.00%> (ø)
src/fastoad/openmdao/problem.py	96.15% <100.00%> (+12.07%)	:arrow_up:

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