[notebook] Fails to render some content when markdown toc extension is enabled

[mirekdlugosz]

On my test website, following pelican configuration:

MARKDOWN = {
    'extension_configs': {
        'markdown.extensions.toc': {'anchorlink': True},
    },
    'output_format': 'html',
}

LIQUID_TAGS = ["notebook"]

causes multiple (but not all) pages to fail with following error message:

ERROR: Could not process pages/site-policy.md
  | append() argument must be xml.etree.ElementTree.Element, not Element
  |___
  | Traceback (most recent call last):
  |   File "/tmp/venv-website/lib/python3.9/site-packages/pelican/generators.py", line 721, in generate_context
  |     page = self.readers.read_file(
  |   File "/tmp/venv-website/lib/python3.9/site-packages/pelican/readers.py", line 573, in read_file
  |     content, reader_metadata = reader.read(path)
  |   File "/tmp/venv-website/lib/python3.9/site-packages/pelican/readers.py", line 337, in read
  |     content = self._md.convert(text)
  |   File "/tmp/venv-website/lib/python3.9/site-packages/markdown/core.py", line 268, in convert
  |     newRoot = treeprocessor.run(root)
  |   File "/tmp/venv-website/lib/python3.9/site-packages/markdown/extensions/toc.py", line 295, in run
  |     self.add_anchor(el, el.attrib["id"])
  |   File "/tmp/venv-website/lib/python3.9/site-packages/markdown/extensions/toc.py", line 218, in add_anchor
  |     c.append(anchor)
  | TypeError: append() argument must be xml.etree.ElementTree.Element, not Element

Not using toc markdown extension or not using notebook liquid tag makes all pages render again, which means that these two sometimes interact in unexpected way.

Root cause should be identified and issue must be fixed, so users are free to use both toc extension and notebook tag on their blogs.