test with different pytest versions

Author: utgwkk

.github/workflows/test.yml

@@ -13,6 +13,12 @@ jobs:
     strategy:
       matrix:
         python-version: [2.7, 3.5, 3.6, 3.7, 3.8]
+        pytest-version: [4.6.11, 5.4.3]
+
+        exclude:
+          # pytest 5.x.x doesn't support Python 2.7
+          - python-version: 2.7
+            pytest-version: 5.4.3
 
     steps:
     - uses: actions/checkout@v2
@@ -25,7 +31,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install -r requirements.txt
+        pip install pytest==${{ matrix.pytest-version }}
 
     - name: Install itself
       run: python setup.py install