CI: Install pyepics 3.5.2, as the newer versions break Python 3.8 env.

.github/workflows/python-app.yml

@@ -26,6 +26,7 @@ jobs:
         python -m pip install --upgrade pip
         pip install flake8 pytest wheel pytest-cov
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+        pip install pyepics==3.5.2
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names