fix: corrected test workflow

the pull_request.yml workflow did not use the requirements_tests.txt file to install the needed requirements

.github/workflows/pull_request.yml

@@ -21,8 +21,8 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        python -m pip install flake8 pytest
-        if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+        python -m pip install flake8
+        pip install -r requirements_tests.txt
     - name: linting
       run: |
         flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics