Group tests with similar duration together

Style and documentation tests take just a few minutes
to run. Since in Github actions one can't restart a single
job but needs to restart an entire workflow, here we group
tests with similar duration together.

.github/workflows/macos_unit_tests.yaml

@@ -11,18 +11,12 @@ on:
       - releases/**
 jobs:
   build:
-
     runs-on: macos-latest
-    strategy:
-      matrix:
-        python-version: [3.7]
-
     steps:
     - uses: actions/checkout@v2
-    - name: Setup Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v2
+    - uses: actions/setup-python@v2
       with:
-        python-version: ${{ matrix.python-version }}
+        python-version: 3.7
     - name: Install Python packages
       run: |
           pip install --upgrade pip six setuptools