ci: clone recursively before creating sdist

otherwise we miss the skia source files

Fixes #40

.github/workflows/ci.yml

@@ -83,14 +83,16 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
+      with:
+        submodules: recursive
     - name: Set up Python
       uses: actions/setup-python@v2
       with:
         python-version: "3.x"
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install setuptools wheel twine
+        pip install --upgrade setuptools wheel twine
     - name: Download artifacts from build jobs
       uses: actions/download-artifact@v2
       with: