feat(core-cython-hello): add sample project using cython-cmake (#43)

Add a minimal scikit-build-core + Cython sample that transpiles a .pyx
via the cython-cmake helpers (find_package(Cython) + UseCython +
cython_transpile), and register it in the noxfile test/dist lists.

Original work by @jcfr and @vyasr; rebased onto main and updated to the
current cython-cmake API.

Assisted-by: ClaudeCode:claude-opus-4.8

Author: jcfr

noxfile.py

@@ -10,6 +10,7 @@
     "hello-cpp",
     "hello-pybind11",
     "hello-cython",
+    "core-cython-hello",
     "hello-cmake-package",
     "pi-fortran",
 ]

projects/core-cython-hello/CMakeLists.txt

@@ -0,0 +1,13 @@
+cmake_minimum_required(VERSION 3.15...3.26)
+
+project(${SKBUILD_PROJECT_NAME} VERSION ${SKBUILD_PROJECT_VERSION} LANGUAGES C)
+
+find_package(
+  Python
+  COMPONENTS Interpreter Development.Module
+  REQUIRED)
+
+find_package(Cython MODULE REQUIRED VERSION 3.0)
+include(UseCython)
+
+add_subdirectory(hello)

projects/core-cython-hello/hello/CMakeLists.txt

@@ -0,0 +1,5 @@
+cython_transpile(_hello.pyx LANGUAGE C OUTPUT_VARIABLE _hello_c)
+
+python_add_library(_hello MODULE "${_hello_c}" WITH_SOABI)
+
+install(TARGETS _hello LIBRARY DESTINATION ${SKBUILD_PROJECT_NAME})

projects/core-cython-hello/hello/__init__.py

@@ -0,0 +1,3 @@
+from ._hello import elevation, hello
+
+__all__ = ("elevation", "hello")

projects/core-cython-hello/hello/_hello.pyx

@@ -0,0 +1,8 @@
+
+cpdef void hello(str strArg):
+    "Prints back 'Hello <param>', for example example: hello.hello('you')"
+    print("Hello, {}!".format(strArg))
+
+cpdef long elevation():
+    "Returns elevation of Nevado Sajama."
+    return 21463

projects/core-cython-hello/pyproject.toml

@@ -0,0 +1,20 @@
+[build-system]
+requires = ["scikit-build-core", "cython>=3.0", "cython-cmake"]
+build-backend = "scikit_build_core.build"
+
+[project]
+name = "hello"
+version = "1.2.3"
+authors = [
+  { name = "The scikit-build team" },
+]
+description = "a minimal example package (cython version)"
+requires-python = ">=3.9"
+classifiers = [
+  "License :: OSI Approved :: MIT License",
+]
+dependencies = []
+
+[tool.scikit-build]
+cmake.source-dir = "."
+minimum-version = "0.5"

projects/core-cython-hello/tests/test_hello_cython.py

@@ -0,0 +1,11 @@
+import hello
+
+
+def test_hello(capfd):
+    hello.hello("World")
+    captured = capfd.readouterr()
+    assert captured.out == "Hello, World!\n"
+
+
+def test_elevation():
+    assert hello.elevation() == 21463