remove build matrix wherever not needed

.github/workflows/tests.yml

@@ -56,11 +56,7 @@ jobs:
   coveralls:
     # check coverage increase/decrease
     needs: tests
-    runs-on: ${{ matrix.os }}
-    strategy:
-      matrix:
-        include:
-          - { os: ubuntu-latest }
+    runs-on: "ubuntu-latest"
     steps:
       - name: Coveralls Finished
         uses: AndreMiras/coveralls-python-action@develop
@@ -69,17 +65,13 @@ jobs:
 
   deploy:
     # builds and publishes to PyPi
-    runs-on: ${{ matrix.os }}
-    strategy:
-      matrix:
-        include:
-          - { python: '3.7', os: ubuntu-latest }
+    runs-on: "ubuntu-latest"
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python
         uses: actions/setup-python@v2
         with:
-          python-version: ${{ matrix.python }}
+          python-version: "3.7"
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip