Fix #6178: i18n: Captions missing in translations for hidden TOCs

This is required to generate inheritance diagrams (as there is now an
example inheritance diagram in the documentation of the extension
generating such diagrams)

The documentation now correctly describes the behavior of the ``parts``
option in an inheritance-diagram directive: it gives the number of parts
that are *kept* not dropped. The option now also accepts negative
values, which drops parts from the left (which is the what the
documentation incorrectly claimed the option would do for positive
values)

As a form of testing of the new functionality, the documentation for the
inheritance_diagram extension now includes a section "Examples" that
demonstrate the different possibilities. This would fail to build
without the patch.

Closes #4872

https://github.com/asottile/pyupgrade

> A tool to automatically upgrade syntax for newer versions of the
> language.

- Drop u str prefix
- Drop base object inheritance
- Drop args to super()
- Use set literals
- Use dict comprehension
- Use set comprehension

Fix #6172: AttributeError is raised for old styled index nodes