Fix tests for python3.9

.github/workflows/main.yml

@@ -39,6 +39,7 @@ jobs:
           "ubuntu-py37-pluggy",
           "ubuntu-py37-freeze",
           "ubuntu-py38",
+          "ubuntu-py39",
           "ubuntu-pypy3",
 
           "macos-py37",
@@ -98,6 +99,10 @@ jobs:
             python: "3.8"
             os: ubuntu-latest
             tox_env: "py38-xdist"
+          - name: "ubuntu-py39"
+            python: "3.8"
+            os: ubuntu-latest
+            tox_env: "py39-xdist"
           - name: "ubuntu-pypy3"
             python: "pypy3"
             os: ubuntu-latest
@@ -133,6 +138,12 @@ jobs:
       uses: actions/setup-python@v1
       with:
         python-version: ${{ matrix.python }}
+    - name: install python3.9
+      if: matrix.tox_env == 'py39-xdist'
+      run: |
+        sudo add-apt-repository ppa:deadsnakes/nightly
+        sudo apt-get update
+        sudo apt-get install -y --no-install-recommends python3.9-dev python3.9-distutils
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip