ci: use matrix in tests workflow to avoid duplication

Use a matrix with the Python version to avoid duplication when testing
different Python versions.

.github/workflows/tests.yml

@@ -40,35 +40,24 @@ jobs:
     - name: Run rule linter
       run: python scripts/lint.py rules/
 
-  tests27:
+  tests:
+    name: Tests in ${{ matrix.python }}
     runs-on: ubuntu-latest
     needs: [code_style, rule_linter]
+    strategy:
+      matrix:
+        include:
+          - python: 2.7
+          - python: 3.8
     steps:
     - name: Checkout capa with submodules
       uses: actions/checkout@v2
       with:
         submodules: true
-    - name: Set up Python 2.7
+    - name: Set up Python ${{ matrix.python }}
       uses: actions/setup-python@v2
       with:
-        python-version: 2.7
-    - name: Install capa
-      run: pip install -e .[dev]
-    - name: Run tests
-      run: pytest tests/
-
-  tests38:
-    runs-on: ubuntu-latest
-    needs: [code_style, rule_linter]
-    steps:
-    - name: Checkout capa with submodules
-      uses: actions/checkout@v2
-      with:
-        submodules: true
-    - name: Set up Python 3.8
-      uses: actions/setup-python@v2
-      with:
-        python-version: 3.8
+        python-version: ${{ matrix.python }}
     - name: Install capa
       run: pip install -e .[dev]
     - name: Run tests