Replace Django 4.1a1 with Django 4.1b1 in the CI matrix

Author: matthiask

tox.ini

@@ -9,7 +9,7 @@ envlist =
 deps =
     dj32: django~=3.2.9
     dj40: django~=4.0.0
-    dj41: django>=4.1a1,<4.2
+    dj41: django>=4.1b1,<4.2
     postgresql: psycopg2-binary
     postgis: psycopg2-binary
     mysql: mysqlclient