Add macOS build helper for Blender 5.1 Python 3.13

[HMiranda00]

Summary

This adds a macOS build helper for Blender 5.1 / Python 3.13, documents the CPython ABI mismatch that prevents Python 3.11 extension modules from loading in Blender 5.1, and adds .skp drag-and-drop import support through Blender's FileHandler API.

Why

Blender 5.1 bundles Python 3.13. When the add-on ships with sketchup.cpython-311-darwin.so, Blender cannot import the native sketchup module. The error shown by Blender can look like a circular import:

cannot import name 'sketchup' from partially initialized module 'sketchup_importer'

The root cause is that the binary extension was compiled for CPython 3.11, while Blender 5.1 imports CPython 3.13 extension modules.

What changed

• Added scripts/build_macos_blender_extension.sh.
• Documented the Blender 5.1 / Python 3.13 macOS build flow in README.md.
• Registered a .skp FileHandler that forwards dropped files to import_scene.skp.
• Reworked imported collection creation so imports are grouped under SKP Imports and a per-file SKP - <filename> collection, instead of relying on shared top-level import collections.
• The helper:
  • uses Blender's bundled Python to derive the extension suffix,
  • downloads matching CPython source headers when Blender does not provide Python.h,
  • supports CPYTHON_SOURCE_DIR for offline/header-cache builds,
  • builds sketchup.cpython-313-darwin.so,
  • rewrites the SketchUpAPI.framework install name to @loader_path,
  • verifies that the built module imports.

Validation

Tested locally on macOS arm64 with Blender 5.1.0 / Python 3.13.9:

Loaded .../sketchup.cpython-313-darwin.so
Built .../sketchup.cpython-313-darwin.so

The resulting module also loaded from Blender's add-on directory after being copied into ~/Library/Application Support/Blender/5.1/scripts/addons/sketchup_importer/.

The Python add-on file was syntax-checked with Blender's bundled Python:

python3.13 -m py_compile sketchup_importer/__init__.py