SlotSchedule scope issue

Caused by removed import in e5b10bb2
Merge branch 'v1.9'

Conflicts:
	indico/core/db/migration.py
Update depedencies
Replace set method with add method

According to Changelog for version 3.6
Replace more occurrences of set method
Make surface_area an optional field
Merge branch 'v1.9'
Remove unused module/class
Code style, missing docstring
Split indico.core.db.sqlalchemy.util into modules

Also move some utils from indico.modules.rb.models.util
Move Serializer to indico.util.serializer
Remove unused function
Rename/document/move double_wraps decorator

It is incredibly useful but now it has a (slightly) better name and
proper documentation. It also is in a more appropriate place.
Move settings model into core
Use a nicer way to import all models
Merge branch 'v1.9'
Merge remote-tracking branch 'upstream/v1.9'
Setup a nice py.test environment

Probably we should get rid of the global temp dir at some point and set
a per-test temp dir from within test cases needing one!
Prevent SMTP/ZODB connections during tests
Add coverage