ci: fix sphinx job

Author: indygreg

.github/workflows/sphinx.yml

@@ -20,7 +20,7 @@ jobs:
           PIP_CONSTRAINT: 'ci/constraints.txt'
         run: |
           uv venv --python 3.13 venv
+          source venv/bin/activate
           uv pip install -r ci/requirements.docs.txt
           uv pip install --config-settings=--build-option=--no-c-backend .
-          source venv/bin/activate
           make -C docs html