[Fix] Fix py3.9 CI (#563)

.github/workflows/build.yml

@@ -116,11 +116,19 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python-version }}
+      # Add ppa source repo for python3.9.
+      - name: Add python3.9 source
+        run: |
+          apt-get update && apt-get install -y software-properties-common
+          add-apt-repository -y ppa:deadsnakes/ppa
+        if: ${{matrix.python-version == '3.9'}}
+      # Install python-dev for some packages which require libpython3.Xm.
+      # Github's setup-python cannot install python3.9-dev, so we have to use apt install.
+      # Set DEBIAN_FRONTEND=noninteractive to avoid some interactions.
       - name: Upgrade pip
         run: python -m pip install pip --upgrade
       - name: Install Python-dev
-        run: apt-get update && apt-get install -y python${{matrix.python-version}}-dev
-        if: ${{matrix.python-version != 3.9}}
+        run: apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends python${{matrix.python-version}}-dev
       - name: Install system dependencies
         run: |
           apt-get update && apt-get install -y ffmpeg libsm6 libxext6 git ninja-build libglib2.0-0 libsm6 libxrender-dev libxext6