drop django 4.0 from CI to avoid security issues (#662)

tox.ini

@@ -1,6 +1,5 @@
 [gh-actions]
 python =
-    3.7: py37
     3.8: py38, apicheck, linkcheck
     3.9: py39, flake8, pydocstyle, cov
     3.10: py310
@@ -10,18 +9,16 @@ python =
 [gh-actions:env]
 DJANGO =
     3.2: django32
-    4.0: django40
     4.1: django41
     4.2: django42
 
 [tox]
 envlist =
-    py37-django{32}
-    py38-django{32,40,41,42}
-    py39-django{32,40,41,42}
-    py310-django{32,40,41,42}
+    py38-django{32,41,42}
+    py39-django{32,41,42}
+    py310-django{32,41,42}
     py311-django{41,42}
-    pypy3-django{32,40,41,42}
+    pypy3-django{32,41,42}
     flake8
     apicheck
     linkcheck
@@ -37,7 +34,6 @@ deps=
     cov: -r{toxinidir}/requirements/test-django.txt
 
     django32: -r{toxinidir}/requirements/test-django32.txt
-    django40: -r{toxinidir}/requirements/test-django40.txt
     django41: -r{toxinidir}/requirements/test-django41.txt
     django42: -r{toxinidir}/requirements/test-django42.txt