🐛 FIX: front-matter rendering with docutils

[chrisjsewell]

For the docutils base HTML writer, it is expected that field_body contain block elements (e.g. paragraph) as children, rather than Text, meaning this file:

---
a: 1
---

leads to:

$ myst-docutils-html test.txt --traceback 
Traceback (most recent call last):
  File "/path/bin/myst-docutils-html", line 33, in <module>
    sys.exit(load_entry_point('myst-parser', 'console_scripts', 'myst-docutils-html')())
  File "/path/lib/python3.7/site-packages/myst_parser/docutils_.py", line 248, in cli_html
    _run_cli("html", "(X)HTML documents", argv)
  File "/path/lib/python3.7/site-packages/myst_parser/docutils_.py", line 242, in _run_cli
    argv=argv,
  File "/path/lib/python3.7/site-packages/docutils/core.py", line 355, in publish_cmdline
    config_section=config_section, enable_exit_status=enable_exit_status)
  File "/path/lib/python3.7/site-packages/docutils/core.py", line 220, in publish
    output = self.writer.write(self.document, self.destination)
  File "/path/lib/python3.7/site-packages/docutils/writers/__init__.py", line 78, in write
    self.translate()
  File "/path/lib/python3.7/site-packages/docutils/writers/_html_base.py", line 80, in translate
    self.document.walkabout(visitor)
  File "/path/lib/python3.7/site-packages/docutils/nodes.py", line 227, in walkabout
    if child.walkabout(visitor):
  File "/path/lib/python3.7/site-packages/docutils/nodes.py", line 227, in walkabout
    if child.walkabout(visitor):
  File "/path/lib/python3.7/site-packages/docutils/nodes.py", line 227, in walkabout
    if child.walkabout(visitor):
  File "/path/lib/python3.7/site-packages/docutils/nodes.py", line 219, in walkabout
    visitor.dispatch_visit(self)
  File "/path/lib/python3.7/site-packages/docutils/nodes.py", line 2021, in dispatch_visit
    return method(node)
  File "/path/lib/python3.7/site-packages/docutils/writers/html4css1/__init__.py", line 414, in visit_field_body
    self.set_class_on_child(node, 'first', 0)
  File "/path/lib/python3.7/site-packages/docutils/writers/_html_base.py", line 407, in set_class_on_child
    child['classes'].append(class_)
TypeError: string indices must be integers

[codecov]

Codecov Report

Merging #477 (56214b0) into master (fb3e8e1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #477      +/-   ##
==========================================
+ Coverage   90.18%   90.19%   +0.01%     
==========================================
  Files          16       16              
  Lines        2027     2030       +3     
==========================================
+ Hits         1828     1831       +3     
  Misses        199      199              

Flag	Coverage Δ
pytests	90.19% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
myst_parser/docutils_renderer.py	92.66% <100.00%> (+0.03%)	⬆️

---

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[chrisjsewell]

TODO: split fixtures test into docutils/sphinx (and make clear they test AST before transforms), add test for docutils, add source/line to elements