Exclude testing py38+py39 with djmain

...to fix the remaining failing tests.
This is due to Django's `main` branch now requiring Python >=3.10.

Example of the errors:
https://github.com/jazzband/django-simple-history/actions/runs/4108104757/jobs/7088396098.

.github/workflows/test.yml

@@ -22,6 +22,12 @@ jobs:
           - python-version: '3.7'
             django-version: 'main'
 
+          - python-version: '3.8'
+            django-version: 'main'
+
+          - python-version: '3.9'
+            django-version: 'main'
+
           - python-version: '3.11'
             django-version: '3.2'
           - python-version: '3.11'

tox.ini

@@ -1,7 +1,8 @@
 [tox]
 envlist =
     py{37,38,39,310}-dj32-{sqlite3,postgres,mysql,mariadb},
-    py{38,39,310}-dj{40,41,main}-{sqlite3,postgres,mysql,mariadb},
+    py{38,39}-dj{40,41}-{sqlite3,postgres,mysql,mariadb},
+    py310-dj{40,41,main}-{sqlite3,postgres,mysql,mariadb},
     py311-dj{41,main}-{sqlite3,postgres,mysql,mariadb},
     docs,
     lint