Fix pp_rules._all_other_rules for pep8.

This doesn't fix **all** pep8 problems with pp_rules.py -- yet :  Only the bits in the recently-split-off function (cf. https://github.com/SciTools/iris/pull/1252).
I have an outstanding PR in this area (https://github.com/SciTools/iris/pull/1239 -- nearly done), and a branch that will become a follow-up PR (https://github.com/pp-mo/iris/tree/all_vector_rules -- checks out, will make PR when the previous is merged).

When those changes come through this file should finally become pep8 compliant.
Meanwhile, I'm posting these changes separately as it removes some irrelevant complexity from  https://github.com/pp-mo/iris/tree/all_vector_rules -- where the changes are otherwise a bit tricky to grasp.