Make kson-lang for Python wheel-only

[holodorum]

kson-lang wraps a native library. The sdist bundled a whole Gradle build so it could try to produce one at install time — a vendored wrapper, a buildSrc copy, MANIFEST.in entries to ship it all, and CI jobs to exercise it.

Making and keeping this working added quite a lot of complexity, instead we decided to make wheels the supported way to install, the sdist is source, not a buid.

What this means for users

Installing needs a published wheel. When none matches, pip falls back to the sdist and it refuses immediately with a pointer to building from source, rather than failing part-way through a Gradle run:

RuntimeError: Cannot build kson-lang for darwin: libkson.dylib, jni_simplified.h missing from .../src/kson.
Those are Gradle output. kson-lang ships prebuilt wheels carrying them, so either none matched this platform and Python version and pip fell back to the source distribution, or this is a checkout where the Gradle build has not run.
Either way, "Build from source" in the readme is the way to get them:
    git clone https://github.com/kson-org/kson.git
    cd kson && ./gradlew :lib-python:build

The sdist still goes to PyPI. It puts the source there; every installable artifact is a wheel.