👌 Nested parse attribution in `attr_block`

[chrisjsewell]

This is in keeping with the restructured text implementation, e.g.

  Hallo

  -- Chris Sewell `link <https://source.com>`__

tokenizes to:

<document source="test.rst">
    <block_quote>
        <paragraph>
            Hallo
        <attribution>
            Chris Sewell 
            <reference name="link" refuri="https://source.com">
                link

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (635d74f) 90.26% compared to head (c7a35c7) 90.26%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #831 +/- ## ======================================= Coverage 90.26% 90.26% ======================================= Files 23 23 Lines 2969 2970 +1 ======================================= + Hits 2680 2681 +1 Misses 289 289 ``` | [Flag](https://app.codecov.io/gh/executablebooks/MyST-Parser/pull/831/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | Coverage Δ | | |---|---|---| | [pytests](https://app.codecov.io/gh/executablebooks/MyST-Parser/pull/831/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | `90.26% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks#carryforward-flags-in-the-pull-request-comment) to find out more.

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