fix(CI): Fix tool invocation with Poetry, add cache action

.github/workflows/python-app.yml

@@ -30,12 +30,22 @@ jobs:
     - name: Install dependencies
       run: |
         poetry install
+    - name: Cache
+      uses: actions/cache@v2.1.6
+      with:
+        # A list of files, directories, and wildcard patterns to cache and restore
+        path: .venv
+        # An explicit key for restoring and saving the cache
+        key: venv-cache-${{hashfiles("poetry.lock")
+        restore-keys: |
+          venv-cache-${{hashfiles("poetry.lock")
+          venv-cache-
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names
-        flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
+        poetry run flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
         # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
-        flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
+        poetry run flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
     - name: Test with pytest
       run: |
-        pytest
+        poetry run pytest