Fix testing on Windows.

.github/workflows/testing.yml

@@ -31,7 +31,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        if [ -f dev-requirements.txt ]; then pip install -r dev-requirements.txt; fi
+        python -m pip install -r dev-requirements.txt
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names