Upgrade python version to >=3.10 and scipy

[pipme]

Updating the Python version to >=3.10 in various workflow files and documentation, upgrading the scipy dependency, and refactoring a gradient checking utility.

Python Version Updates:

• .github/workflows/build.yml: Updated Python version from 3.9 to 3.10.
• .github/workflows/merge-tests.yml: Updated supported Python versions to 3.10, 3.11, and 3.12.
• .github/workflows/tests.yml: Updated supported Python versions to 3.10, 3.11, and 3.12.
• README.md: Updated Python version requirement to 3.10 or newer.
• docsrc/source/development.rst: Updated Python version target to 3.10 and up.
• docsrc/source/installation.rst: Updated Python version requirement to 3.10 or newer.

Dependency Updates:

• pyproject.toml: Upgraded scipy dependency to version 1.15.0 and updated the required Python version to 3.10 or newer.

Codebase Improvements:

• pyvbmc/testing/_check_grad.py: Refactored gradient checking utility to use scipy.differentiate.jacobian instead of scipy.misc.derivative.
• pyvbmc/testing/entropy/test_entmc_vbmc.py: Modified initialization of vp.sigma, vp.lambd, and vp.eta to ensure positive values and fixed vp.eta initialization.