Pin pytest-django to <3.2.0 while we're still not on Django 1.11

* pytest-django 3.2.0 dropped Django 1.8 support

Refs https://github.com/mozilla/addons-server/pull/8045

requirements/dev.txt

@@ -33,7 +33,7 @@ pytest-cache==1.0 \
     --hash=sha256:be7468edd4d3d83f1e844959fd6e3fd28e77a481440a7118d430130ea31b07a9
 pytest-django==3.1.2 \
     --hash=sha256:038ccc5a9daa1b1b0eb739ab7dce54e495811eca5ea3af4815a2a3ac45152309 \
-    --hash=sha256:00995c2999b884a38ae9cd30a8c00ed32b3d38c1041250ea84caf18085589662
+    --hash=sha256:00995c2999b884a38ae9cd30a8c00ed32b3d38c1041250ea84caf18085589662 # pyup: <3.2.0
 pytest-xdist==1.22.2 \
     --hash=sha256:be2662264b035920ba740ed6efb1c816a83c8a22253df7766d129f6a7bfdbd35 \
     --hash=sha256:e8f5744acc270b3e7d915bdb4d5f471670f049b6fbd163d4cbd52203b075d30f