Collect coverage data when rendering widgets.

[schinckel]

Resolves #94

[schinckel]

Okay, this doesn't actually collect coverage on real code.

It seems to be because line_number_range(frame) bails out if the self object is an instance of django.template.base.Template, which the template used for rendering widgets is, even when using the django template backend/engine.

@nedbat do you recall why this is?

https://github.com/nedbat/django_coverage_plugin/blob/ca45852f7168d36b69e6a64f01d804b572775680/django_coverage_plugin/plugin.py#L233

[schinckel]

Ah, think I figured that out - it's the nodes within a template that have the line numbers available.

The issue is that form templates use a different rendering thing, which is in a different python file, so is being omitted by file_tracer.

[schinckel]

Ah, turns out it needs settings.FORM_RENDERER to be set to 'django.forms.renderers.TemplatesSetting'