Bundle PR on a bi-weekly basis (#757)

Currently pyup causes a lot of noise so when looking at the closed
PR the real PRs can be easily overlooked.
As we only have test dependencies simply updating on a bi-weekly basis
should be sufficient.

.pyup.yml

@@ -1,4 +1,5 @@
 search: False
+schedule: "every two weeks"
 requirements:
   - requirements/requirements-codestyle.txt:
       update: all