CI: Install all requirements at the same time

.github/workflows/ci.yml

@@ -51,12 +51,12 @@ jobs:
         python -m pip install --upgrade pip
         pip install numpy wheel
         pip install -r requirements.txt
+        pip install -r dev-requirements.txt
 
     # Note the use of the -Wa flag to show DeprecationWarnings
     # We run the tests on the installed package
     - name: Unit tests
       run: |
-        pip install -r dev-requirements.txt
         python setup.py install
         cd ~
         python -Wa -m pytest --pyargs skued --import-mode=importlib