Try to fix GitHub actions workflow

Author: adamtheturtle

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
       matrix:
         # These versions match the minimum and maximum versions of pip in
         # requirements.txt
-        extra-pip-version-to-install: ['==10.0.1', '<= 19']
+        pip-version: ['==10.0.1', '<= 19']
         python-version: [3.6, 3.7, 3.8]
 
     steps:
@@ -30,7 +30,7 @@ jobs:
       - name: "Install dependencies"
         run: |
           pip install --upgrade --editable .[dev]
-          pip install --upgrade 'pip ${{ extra-pip-version-to-install }}'
+          pip install --upgrade 'pip ${{ matrix.pip-version }}
           pip install flake8
 
       - name: "Lint"