Use virtualenv to isolate from VM changes

e.g. update of 20200709 breaks dependencies with PyYAML
because new package aws-sam-cli has ben added
487c88b726

.github/workflows/flask.yml

@@ -1,4 +1,4 @@
-name: Django CI
+name: Flask CI
 
 on:
   push:
@@ -46,6 +46,9 @@ jobs:
         sudo apt-get update -qy
         sudo apt-get -y install postgresql-client
         python -m pip install --upgrade pip
+        pip install virtualenv
+        virtualenv env
+        source env/bin/activate
         pip install flake8 pytest
         pip install -r requirements.txt
 
@@ -63,5 +66,6 @@ jobs:
 
     - name: Run Tests
       run: |
+        source env/bin/activate
         pytest -m mvp --maxfail=5 tests/