Install python.

.github/workflows/pythonwheels.yml

@@ -79,6 +79,10 @@ jobs:
     needs: build
     if: github.event_name == 'push' && startsWith(github.ref, 'refs/tags/dulwich-')
     steps:
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v2
+      with:
+        python-version: "3.x"
     - name: Install twine
       run: |
         python -m pip install --upgrade pip