ogp_custom_meta_tags now warns if it is a list

[nijel]

Before 171a80b ogp_custom_meta_tags was supposed to be a list to not trigger a warning. Since then it is supposed to be a tuple. That causes configuration that used to work to end up with:

WARNING: The config value `ogp_custom_meta_tags' has type `list', defaults to `tuple'.

As we run Sphinx with --fail-on-warning, the upgrade to 0.11.0 breaks the build. It is easy to fix, but it would be great to keep the existing configurations to work.

[m-aciek]

It should have been fixed by 4b268cd.