CI: use Python 3.14 for free threading tests

[dweindl]

The latest scipy doesn't provide wheels for py313t, so let's use py314t.

(Avoids building scipy ourselved or using python-version-specific scipy requirements.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.42%. Comparing base (fc6a1c5) to head (e85af65).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3191      +/-   ##
==========================================
- Coverage   78.09%   77.42%   -0.67%     
==========================================
  Files         317      317              
  Lines       20903    20903              
  Branches     1483     1483              
==========================================
- Hits        16324    16184     -140     
- Misses       4571     4711     +140     
  Partials        8        8              

Flag	Coverage Δ
cpp	71.56% <ø> (-0.04%)	⬇️
cpp_python	36.71% <ø> (ø)
petab	46.78% <ø> (ø)
petab_sciml	15.54% <ø> (ø)
python	68.31% <ø> (-0.04%)	⬇️
sbmlsuite-jax	?

Flags with carried forward coverage won't be shown. Click here to find out more.
see 5 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.