Fix compatibility with Pytest 8.4+

[lkubb]

What does this PR do?

• Fixes test suite installation of Salt (see [BUG] pypi packages missing wheels and/or dependencies salt#67973)
• Accounts for new Pytest internals in 8.4, fixing compatibility with it.

What issues does this PR fix or reference?

Fixes: #187

Previous Behavior

Any test run with pytest-salt-factories and pytest>=8.4 crashes hard

New Behavior

Works as expected

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Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.77%. Comparing base (025dd47) to head (26d1fb8).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
src/saltfactories/plugins/loader.py	87.50%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
+ Coverage   85.27%   85.77%   +0.51%     
==========================================
  Files          97       97              
  Lines        5977     5973       -4     
  Branches      636      635       -1     
==========================================
+ Hits         5096     5123      +27     
+ Misses        705      673      -32     
- Partials      176      177       +1     

Flag	Coverage Δ
Linux	85.24% <87.50%> (+0.53%)	⬆️
Windows	77.04% <87.50%> (+0.68%)	⬆️
macOS	75.49% <87.50%> (+0.44%)	⬆️
py3.10	85.51% <87.50%> (+0.58%)	⬆️
py3.7	?
py3.8	?
py3.9	?
pytest-7.4.0	85.44% <87.50%> (+0.46%)	⬆️
pytest-8.0.0	84.70% <87.50%> (+0.40%)	⬆️
pytest-8.1.0	?
pytest-8.1.1	84.80% <87.50%> (?)
pytest-8.2.0	84.84% <87.50%> (?)
pytest-8.3.3	84.82% <87.50%> (?)
salt-3006.0	?
salt-3006.9	85.37% <87.50%> (?)
salt-3007.0	?
salt-3007.1	84.75% <87.50%> (?)
src	85.51% <87.50%> (+8.23%)	⬆️
tests	96.28% <ø> (-0.08%)	⬇️

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Files with missing lines	Coverage Δ
src/saltfactories/plugins/loader.py	95.66% <87.50%> (-4.34%)	⬇️

... and 10 files with indirect coverage changes

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[lkubb]

Afaict, the remaining failures are caused by the same underlying issue that is fixed by the first commit in this PR, but in the ghcr.io/saltstack/salt-ci-containers/salt:3006 container.

This needs to be fixed in https://github.com/saltstack/salt-ci-containers by constraining setuptools<75.6.0 during Salt installation in the Dockerfile. Edit: saltstack/salt-ci-containers#109