Document context processor removal in upgrade docs

[Osmose]

The context processor we used to need is no longer required, but the upgrade docs don't mention removing it. This can drive you insane if you hit some specific conditions:

• You install django_browserid in a way that removed files won't get deleted (like using pip on vendor-local in a playdoh project).
• You neglect to remove the context processor.

This causes browserid_info to fail because the context processor partially applies an argument to it, when the current version takes no arguments. Good lord that was a pain to find.

[peterbe]

How about adding it back in and make it do nothing other than a deprecation warning?

Relying on reading docs is suboptimal.

[willkg]

I think in this case, since the file is gone in the new version, it's not just an issue of changing something--it's no longer there and thus will raise errors by the fact it's no longer there. However, there are a few edge cases (which I'll call pathological installation methods) where the file sticks around from previous versions. I don't think that affects most people. Most people should be installing from pypi or at least being careful to update properly. If they're not, I contend they have other bigger issues.

Given that, I think I'm +1 on documenting, but -1 on adding code to deal with this.

[willkg]

Also, in regards to playdoh, the maintainer docs specifically state to remove all the files before pip installing a new version:

http://playdoh.readthedocs.org/en/latest/maintenance/vendor.html#updating-a-library-via-pip

[peterbe]

So, basically either you get an ImproperlyConfigured error or you get context_processor.pyc running and ghosting about.

Fine, a little blurb in the Troubleshooting section then about deleting *.pyc files should suffice.