Make template system backward compatible

[maartenbreddels]

Fixes #1158

This also makes nbconvert 6.0.0 compatible with voila (0.1.20 as of writing), and I guess many more projects.

The basic idea is to add templates with the old names in the compatibility subdirectory, which will be added to the search path. I've done this for rst.tpl and display_priority.tpl, but if people like this I can do it for the other old template filenames as well.

I think it does not bring an extra dev burden for this project while staying backward compatible.

Should we try to make it fully backward compatible, including running tests?

[MSeal]

So I would like to not indefinitely support old patterns if we can avoid it. I think this is a good idea for the 6.0.0 release but it'd be nice if we could emit a deprecation warning stating the new path they should be using. Thoughts?

[MSeal]

Pinging to refresh inboxes on PR conversation

[maartenbreddels]

So I would like to not indefinitely support old patterns if we can avoid it.

It would make many tools just work, it's 1 lines per template, I think it would make library owners happy.

We could add the templates, indeed, with a warning, but only remove it in version 7?

Or we do the most popular only? html/rst?

[MSeal]

It would make many tools just work, it's 1 lines per template, I think it would make library owners happy.

I agree

We could add the templates, indeed, with a warning, but only remove it in version 7?

If it's easy to add the warning and have an extra line or two per template that does sound ideal. Maybe we should pick a release sooner than 7 to remove it (who know how many years that is)? 6.2? 6.3? Or maybe we just say no sooner than 6.2 to leave it open to extending removal?

[maartenbreddels]

I would say, we keep it to this, and we can add support for more old template names on a request/PR basis, using this as an example PR. This gives us some feedback on how people rely on it, and how they feel about migrating.

[MSeal]

Works for me -- I'm going to get #1211 merged and I think we can do a new alpha release. Any other changes you think we need to address before doing so?

[MSeal]

Also docs build needs a fix -- some linting error in sphinx that's unrelated to this