fix: set PYTHONPATH at job level so all steps can import build_backend

Step-level env only applies to a single step. uv sync passed but uv build
and tox steps still failed with ModuleNotFoundError: No module named build_backend.

Moving PYTHONPATH to job-level env ensures it is available for all steps:
uv sync, uv build, tox, and any future steps that invoke the build backend.

Also fix docstring example in build_backend.py to include packaging in
build-system.requires, matching what the packages actually declare.

Author: sellakumaran

.github/workflows/ci.yml

@@ -85,6 +85,8 @@ jobs:
     defaults:
       run:
         working-directory: ./
+    env:
+      PYTHONPATH: ${{ github.workspace }}/versioning/helper
 
     strategy:
       matrix:
@@ -105,8 +107,6 @@ jobs:
 
     - name: Install the project
       run: uv lock && uv sync --locked --all-extras --dev
-      env:
-        PYTHONPATH: ${{ github.workspace }}/versioning/helper
 
     - name: Verify centralized version constraints
       run: uv run --frozen tox -e verify-constraints

versioning/helper/build_backend.py

@@ -10,7 +10,7 @@
 
 Usage in package pyproject.toml:
   [build-system]
-  requires = ["setuptools>=68", "wheel", "tzdata", "tomlkit"]
+  requires = ["setuptools>=68", "wheel", "tzdata", "tomlkit", "packaging"]
   build-backend = "build_backend"
 
 The build backend module is resolved via PYTHONPATH, which must include the