I was a little too bash script for powershell :-)

.github/workflows/build.yml

@@ -75,7 +75,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install flake8 pytest
-        if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+        pip install -r requirements.txt
         pip install .
     - name: Test with pytest
       run: |