tox: pin max supported django version to v3.2.x

Django made a new release v4.0 supporting python3.8 and 3.9 today. This
breaks some test cases and need further inspectation.

See: #443
Signed-off-by: You-Sheng Yang <vicamo@gmail.com>

tox.ini

@@ -14,7 +14,7 @@ deps =
     django31: django~=3.1.0
     django31: djangorestframework~=3.12.0
     django31: django-filter~=21.1.0
-    django32: django>=3.2.0
+    django32: django~=3.2.0
     django32: djangorestframework~=3.12.0
     django32: django-filter~=21.1.0
 setenv =