ci: Test only latest Python runtime on MacOS (#1203)

* Exclude Python 3.6 and Python 3.7 from the testing matrix for macos-latest
   - Speed up CI significantly while still testing the latest Python runtime on macos-latest
* Add testing of Python 3.6 to current release tests
* c.f. https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-syntax-for-github-actions#example-excluding-configurations-from-a-matrix

Author: matthewfeickert

.github/workflows/ci.yml

@@ -16,6 +16,11 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest]
         python-version: [3.6, 3.7, 3.8]
+        exclude:
+          - os: macos-latest
+            python-version: 3.6
+          - os: macos-latest
+            python-version: 3.7
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/release_tests.yml

@@ -15,7 +15,12 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest]
-        python-version: [3.7, 3.8]
+        python-version: [3.6, 3.7, 3.8]
+        exclude:
+          - os: macos-latest
+            python-version: 3.6
+          - os: macos-latest
+            python-version: 3.7
 
     steps:
     - uses: actions/checkout@v2