Speed up CI by reducing the depth of linting/type checking

This keeps the bottom and top versions, which is likely to be sufficient.

Author: PeterJCLaw

.github/workflows/check.yml

@@ -49,15 +49,12 @@ jobs:
 
     strategy:
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9"]
-        django-version: ["2.2", "3.0", "3.1", "3.2", "4.0"]
+        python-version: ["3.6", "3.9"]
+        django-version: ["2.2", "4.0"]
         exclude:
           # Python 3.6 is not supported beyond Django 3.2
           - python-version: "3.6"
             django-version: "4.0"
-          # Python 3.7 is not supported beyond Django 3.2
-          - python-version: "3.7"
-            django-version: "4.0"
 
     steps:
       - name: Checkout
@@ -88,15 +85,12 @@ jobs:
 
     strategy:
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9"]
-        django-version: ["2.2", "3.0", "3.1", "3.2", "4.0"]
+        python-version: ["3.6", "3.9"]
+        django-version: ["2.2", "4.0"]
         exclude:
           # Python 3.6 is not supported beyond Django 3.2
           - python-version: "3.6"
             django-version: "4.0"
-          # Python 3.7 is not supported beyond Django 3.2
-          - python-version: "3.7"
-            django-version: "4.0"
 
     steps:
       - name: Checkout