Fix cmake and Cython compile issues; update GH Action CI; extend support for python 3.12, 3.13, 3.14

[mtazzari]

Closes #200 and addresses #201.

Build and compatibility

• Raise the project CMake minimum to 3.10 so CMake 4 can configure the project.
• Update the Cython array-base handling for Cython 3.
• Update test helpers for NumPy 2 and current SciPy.
• Replace the distutils-dependent PythonInstall helper with a Python-standard-library install-path calculation, allowing Python 3.12+.
• Replace the legacy CMake pytest helper while retaining build/python/py.test.sh for manual test commands.

Linux installation coverage

• Add a pip-based Linux/amd64 Docker harness for future PyPI-oriented coverage.
• Add a Conda/Miniconda Linux/amd64 Docker harness that mirrors the documented and GitHub Actions build path.
• Add build helpers for both images and document when to use each.

CI and documentation

• Move GitHub Actions from the retired Ubuntu 20.04 runner to Ubuntu 24.04.
• Update checkout to v6 and setup-miniconda to v4.
• Extend the matrix with Python 3.12, 3.13, and 3.14; retain the existing 3.7 to 3.9 coverage.
• Remove unused astropy and the legacy tar.bz2 Conda package restriction.
• Replace the deprecated Codecov curl uploader with codecov-action v5; uploads are non-blocking and use the CODECOV_TOKEN secret.
• Update the Sphinx CSS API and language setting for current Sphinx.

Validation

• pip Docker image, linux/amd64: C++ test, installed-package import, and all 35 Python tests passed.
• Conda Docker image, linux/amd64, Python 3.12: C++ test, installation into the active Conda environment, installed-package import, and all 35 Python tests passed.
• The GitHub Actions matrix validates the additional Python versions.

CUDA

CUDA configuration and GPU targets are unchanged. The Docker harnesses intentionally configure CPU-only builds because they run without an NVIDIA toolkit or GPU. GPU compilation remains to be validated on a compatible NVIDIA runner.

Follow-up

The legacy FindCython.cmake and UseCython.cmake modules can still emit CMP0148 policy warnings with recent CMake; modernizing those modules is intentionally deferred.

[mtazzari]

The reported GreatCMakeCookOff PythonInstall.cmake CMP0148 warning when running cmake

...
-- Building the galario python wrapper
CMake Warning (policy) at build/external/src/GreatCMakeCookOff/scripts/PythonInstall.cmake:3 (find_package):
  Policy CMP0148 is not set: The FindPythonInterp and FindPythonLibs modules
  are removed.  Run "cmake --help-policy CMP0148" for policy details.  Use
  the cmake_policy command to set the policy and suppress this warning.
Call Stack (most recent call first):
  python/CMakeLists.txt:52 (include)
This warning is for project developers.  Use -Wno-author or -Wno-policy to
suppress it.
...

comes from the current master version of python/CMakeLists.txt, which still includes the legacy PythonInstall helper. This PR replaces that helper with a Python-standard-library installation-path calculation, removing both that warning source and the distutils dependency that blocks Python 3.12+.

Separate CMP0148 warnings may remain from the legacy FindCython.cmake and UseCython.cmake modules. They are not blockers for this PR.