installation: requirement addition for six library

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
arxiv: docs entry addition

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
docs: css theme overrides

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
registry: keygetter value fix

* `register` method of `DictModuleAutoDiscoverySubRegistry` class
  gives original value of parameter `value` to the keygetter method
  as well as the modified value from valuegetter.

* NOTE If your registry.py is using DictModuleAutoDiscoverySubRegistry
  and you use your own keygetter method then please update your
  keygetter method to use all three parameters.

Co-authored-by: Jan Åge Lavik <jan.age.lavik@cern.ch>
Co-authored-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Signed-off-by: Graham R. Armstrong <graham.richard.armstrong@cern.ch>
workflows: registry fix for keygetter method

* `keygetter` method of WorkflowsRegistry updated for keygetter fix.

Co-authored-by: Jan Åge Lavik <jan.age.lavik@cern.ch>
Signed-off-by: Graham R. Armstrong <graham.richard.armstrong@cern.ch>