LMFIT: Non-Linear Least-Squares Minimization and Curve-Fitting for Python

Fixes:

• fix loading spline models with more than five knots (Issue #985)
• improved SplineModel to explicitly allow more knots, make it easier to evaluate and save/reload (Issue #985 PR #989, Paul Müller)
• improvements to adding Parameters.
• support Model functions with "barestar" syntax (PR #982)
• fix several related problems with providing a Jacobian function, especially for consistency across solvers (including least_squares), and for pickling (PR #973 Ville Yrjänä)
• fix Step and Rectangle Models to allow a negative value for sigma, indicating a downward step (PR #970)

Build, Maintenance:

• update issue templates
• add test for consistent init_fit and best_fit for saved/looded SplineModel
• fix NumPy v2 DeprecationWarning
• uppdate SciPy/NumPy dependencies.
• remove numexpr dependency (again)
• drop support for Python 3.8, add Python 3.13
• asteval no longer raises NameError to Python, so we catch exceptions from asteval when creating parameters.
• avoid setting stderr to None for uncertainties calculations.

Documentation and Examples:

• add example with uncertainties in both x and y (#992)
• make sign of residual calculations in model.py consistent with documentation (Discussion #986, PR#987, Timothy-J-Warner)
• add example fitting multiple datasets using Model interface (Discussion #904, PR #967, mstekiel)
• tweaks to 'sphinx-gallery' settings.
• update names of the documentation examples in Gallery