Issue when captioning multiple figures that are produced by code

[mmcky]

Describe the problem

When using caption and labels in the cell metadata, there is an issue when there is more than one figure in a single document

self = <document: <section "formatting code outputs"...>>, node = <figure "fun-fish2": <image...><caption...>>, id = 'fun-fish2'
msgnode = <figure "fun-fish2": <image...><caption...>>, explicit = True

    def set_name_id_map(self, node, id, msgnode=None, explicit=None):
        """
        `self.nameids` maps names to IDs, while `self.nametypes` maps names to
        booleans representing hyperlink type (True==explicit,
        False==implicit).  This method updates the mappings.
    
        The following state transition table shows how `self.nameids` ("ids")
        and `self.nametypes` ("types") change with new input (a call to this
        method), and what actions are performed ("implicit"-type system
        messages are INFO/1, and "explicit"-type system messages are ERROR/3):
    
        ====  =====  ========  ========  =======  ====  =====  =====
         Old State    Input          Action        New State   Notes
        -----------  --------  -----------------  -----------  -----
        ids   types  new type  sys.msg.  dupname  ids   types
        ====  =====  ========  ========  =======  ====  =====  =====
        -     -      explicit  -         -        new   True
        -     -      implicit  -         -        new   False
        None  False  explicit  -         -        new   True
        old   False  explicit  implicit  old      new   True
        None  True   explicit  explicit  new      None  True
        old   True   explicit  explicit  new,old  None  True   [#]_
        None  False  implicit  implicit  new      None  False
        old   False  implicit  implicit  new,old  None  False
        None  True   implicit  implicit  new      None  True
        old   True   implicit  implicit  new      old   True
        ====  =====  ========  ========  =======  ====  =====  =====
    
        .. [#] Do not clear the name-to-id map or invalidate the old target if
           both old and new targets are external and refer to identical URIs.
           The new target is invalidated regardless.
        """
        for name in node['names']:
            if name in self.nameids:
                self.set_duplicate_name_id(node, id, name, msgnode, explicit)
            else:
                self.nameids[name] = id
>               self.nametypes[name] = explicit
E               TypeError: 'dict_items' object does not support item assignment

MyST-NB/.tox/py38-sphinx4/lib/python3.8/site-packages/docutils/nodes.py:1465: TypeError

Link to your repository or website

https://github.com/executablebooks/MyST-NB/pull/373

Steps to reproduce

Added a test case

https://github.com/executablebooks/MyST-NB/pull/373

The version of Python you're using

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Your operating system

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Versions of your packages

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Additional context

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