reStructuredText literal blocks within other directives

### Python Version

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### Package Version

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### Description

The new `--rst-literal-blocks` option from #196 picks up literal blocks *within* other directives like code blocks:

```
How to do literal blocks:

.. code-block:: rst

    A literal block::

        This is not Python.
```

Ideally these should be skipped, like nested code blocks in #128.