Add python 3.9 to tox tests

.github/workflows/tox.yml

@@ -4,22 +4,26 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.8]
+        python-version: [3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v2
+
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
+
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
         pip install tox poetry
+        # Following step is required for python 3.9: Build depenencies are missing
+        sudo apt-get update && sudo apt-get install libxml2-dev libxslt-dev python3-dev -y
 
     - name: Run linters in tox
       run: tox -e linters