Update python-app.yml for Windows-bash

Author: Ahmad-Waseem

.github/workflows/python-app.yml

@@ -26,7 +26,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install flake8 pytest
-        if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+        if (Test-Path requirements.txt) { pip install -r requirements.txt }
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names