Tell PyUp to look at .in files

We only want PyUp to be checking for the dependencies we specify
directly, not any sub-dependencies.

By telling it to now look only at `.in` files we preserve this existing
behaviour.

.pyup.yml

@@ -4,5 +4,5 @@ schedule: "every week on monday"
 
 search: False
 requirements:
-  - requirements-app.txt
-  - requirements-dev.txt
+  - requirements.in
+  - requirements_for_test.in