Fix Python version in GH Actions CI/CD pipeline

[juhoinkinen]

All test jobs were running tests on Python 3.10 although the intent was to use the versions defined in the job configurations matrix, i.e. 3.8, 3.9 and 3.10. This was noticeable only(?) in the output of the pytest step, which for all jobs was

platform linux -- Python 3.10.6, pytest-7.4.0, pluggy-1.2.0

(And from noticing that everything worked after adding Python 3.11 to the matrix (#703), which was because 3.10 was actually used.)

Although the "Set up Python" step installed the Python versions defined in the matrix as requested, Poetry was installed with pipx, which in turn seemed to be (pre-)installed on Python 3.10, and this version remained to be used in the Poetry venv.

Now the Python version from the matrix is set to be used in Poetry venv by poetry env use <Python-version> command.

Also upgrades Poetry to v1.5.1.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (98603a9) 99.67% compared to head (1442f03) 99.67%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #717   +/-   ##
=======================================
  Coverage   99.67%   99.67%           
=======================================
  Files          89       89           
  Lines        6380     6380           
=======================================
  Hits         6359     6359           
  Misses         21       21           

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[sonarqubecloud]

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