Defer check for valid django.template.backends.django.DjangoTemplates

[PamelaM]

Until django.template.backends.django has been imported.

[nedbat]

Nice commit message :) I made you a collaborator if you want to merge these yourself. Looks good to me: 👍

[PamelaM]

Classic working at home commit message:

1. Plan to type out "Fix bug that... etc"
2. Type "Fix bug"
3. Get interrupted
4. Hit return
5. Say "D'oh!" :)

[PamelaM]

This will fix the immediate issue (the travis tests pass, except for django10tip because someone checked in a py3-only reference to lru_cache). So I'm merging.

I think we can create a new release that will fix most of the normal cases, but might still cause problems with projects that don't use DjangoTemplates but still use some other templating system.

I've got a test_project kind of in place, but I want to automate it's creation so that it will work with all supported django versions. I need to add a testing-only dependency on jinja2 or some even more obscure template engine to confirm that the plugin works in the face of django.template.backends.django never getting imported, but a template file attempts to get coverage, etc.

[nedbat]

Find me on IRC if you want to talk through any of that stuff. If not, charge on! :)