⬆️ Drop Sphinx<7.4, test against Python 3.13

[ubmarco]

• Sphinx 7.4 is needed for the write-started event, which is now used to ensure needs post-processing is always run on incremental builds (via a call to get_needs_view())
• Expanded Python to 3.13. Keeping also 3.12 in the CI as it behaves differently for os.path.isabs.
• Updated test_app fixture to accept a files request parameter to parametrize the src file generation.
• Use the self.env.relfn2path() Sphinx function for needimport needs.json paths.
• Added test cases for needimport that also include absolute paths for all OS.

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This is preliminary work for #1441 in an isolated PR.

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.54%. Comparing base (4e10030) to head (8939f80).
Report is 127 commits behind head on master.

Files with missing lines	Patch %	Lines
sphinx_needs/directives/needbar.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1447      +/-   ##
==========================================
+ Coverage   86.87%   88.54%   +1.66%     
==========================================
  Files          56       60       +4     
  Lines        6532     7243     +711     
==========================================
+ Hits         5675     6413     +738     
+ Misses        857      830      -27     

Flag	Coverage Δ
pytests	88.54% <94.73%> (+1.66%)	⬆️

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[ubmarco]

CI fails for Windows only for Python 3.13. Need help or time investigating this.

[ubmarco]

Windows problem fixed

[chrisjsewell]

see comments

[ubmarco]

@chrisjsewell please review this again