Update run_tests.yml

Add pyenv action to support Python 3.5.3

Author: bitkeks

.github/workflows/run_tests.yml

@@ -16,9 +16,9 @@ jobs:
       - uses: actions/checkout@v2
 
       - name: Set up Python 3.5.3
-        uses: actions/setup-python@v2
+        uses: gabrielfalcao/pyenv-action@v7
         with:
-          python-version: '3.5.3'  # Debian Buster (stable)
+          default: '3.5.3'  # Debian Buster (stable)
 
       - name: Run Python unittests
         run: python3 -m unittest