3.6: RST "Reference not found", used to work in 3.5

[nedbat]

One form of .rst link that worked in 3.5 is no longer working in 3.6.

In foo.rst, "pull 234" isn't found as a reference:

- One issue fixed: `issue 123`_.

- One change merged: `Big change <pull 234_>`_.

- Improved the `home page <https://example.com/homepage>`_.

- One more `small change`__.

.. _issue 123: https://github.com/joe/project/issues/123
.. _pull 234: https://github.com/joe/project/pull/234
__ https://github.com/joe/project/issues/999

With 3.5:

% /dwn/pandoc-3.5-arm64/bin/pandoc -frst -tmarkdown_strict foo.rst
-   One issue fixed: [issue
    123](https://github.com/joe/project/issues/123).
-   One change merged: [Big
    change](https://github.com/joe/project/pull/234).
-   Improved the [home page](https://example.com/homepage).
-   One more [small change](https://github.com/joe/project/issues/999).

With 3.6:

% /dwn/pandoc-3.6-arm64/bin/pandoc -frst -tmarkdown_strict foo.rst
[WARNING] Reference not found for 'pull 234' at foo.rst_chunk line 1 column 45
-   One issue fixed: [issue
    123](https://github.com/joe/project/issues/123).
-   One change merged: [Big change]().
-   Improved the [home page](https://example.com/homepage).
-   One more [small change](https://github.com/joe/project/issues/999).