change pip to pip3 in install depenencies section of pythonapp workflow

.github/workflows/pythonapp.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install -r requirements.txt
+        pip3 install -r requirements.txt
     - name: Lint with flake8
       run: |
         pip install flake8