test: django tip dropped support for 3.8 and 3.9

I'm not sure why they did, but they did.

Author: nedbat

tox.ini

@@ -15,8 +15,8 @@
 [tox]
 envlist =
     py37-django{111,22,32}-cov{5,6,tip},
-    py38-django{22,32,40,41,tip}-cov{5,6,tip},
-    py39-django{22,32,40,41,tip}-cov{5,6,tip},
+    py38-django{22,32,40,41}-cov{5,6,tip},
+    py39-django{22,32,40,41}-cov{5,6,tip},
     py310-django{32,40,41,tip}-cov{5,6,tip},
     py311-django{41,tip}-cov{6,tip},
     check,pkgcheck,doc