Simplified tox envlist

There's now just one Django version per line - similar to the tables in
`README.rst` and `docs/index.rst`.

Author: ddabble

tox.ini

@@ -1,9 +1,9 @@
 [tox]
 envlist =
     py{38,39,310}-dj32-{sqlite3,postgres,mysql,mariadb},
-    py{38,39}-dj{40,41}-{sqlite3,postgres,mysql,mariadb},
-    py310-dj{40,41,main}-{sqlite3,postgres,mysql,mariadb},
-    py{311,312}-dj{41,main}-{sqlite3,postgres,mysql,mariadb},
+    py{38,39,310}-dj40-{sqlite3,postgres,mysql,mariadb},
+    py{38,39,310,311,312}-dj41-{sqlite3,postgres,mysql,mariadb},
+    py{310,311,312}-djmain-{sqlite3,postgres,mysql,mariadb},
     docs,
     lint