fix: Fix permissions error in pipeline when generating Python runtime

Author: dstufft

packages/jsii-python-runtime/package.json

@@ -4,7 +4,7 @@
   "description": "Python client for jsii runtime",
   "main": "index.js",
   "scripts": {
-    "generate": "bin/generate",
+    "generate": "python3 bin/generate",
     "deps": "python -m venv .env && .env/bin/pip install pip==19.0.1 setuptools==40.7.0 wheel==0.32.3 && .env/bin/pip install -r requirements.txt",
     "build": "npm run generate && npm run deps && .env/bin/python setup.py sdist -d . bdist_wheel -d . && rm -rf build",
     "package": "package-python",