Use markdown-it-py to render Markdown content (#28)

Replace the MyST parser, which actually uses markdown-it-py. MyST is
meant to be used with sphinx and didn't really offer that much in the
way of customizing the Markdown rendering process. With markdown-it-py
we can use some of their plugins and even write some of our own to
support things like syntax highlighting for code blocks (with language
specification, like in GitHub flavored markdown). For now, just replace
the rendering and add a few plugins (typographic substitutions,
footnotes, tables, and anchors for headings.

environment.yml

@@ -4,7 +4,7 @@ channels:
   - conda-forge
   - defaults
 dependencies:
-  - python==3.9
+  - python==3.10
   - pip
   - make
   - pip:

nene/rendering.py

@@ -3,7 +3,9 @@
 # SPDX-License-Identifier: MIT
 """Render outputs with Jinja templates."""
 import jinja2
-import myst_parser.main
+import mdit_py_plugins.anchors
+import mdit_py_plugins.footnote
+from markdown_it import MarkdownIt
 
 
 def make_jinja_env(templates_dir):
@@ -43,10 +45,29 @@ def markdown_to_html(page):
         The converted HTML.
 
     """
-    html = myst_parser.main.to_html(page["markdown"])
+    parser = MarkdownIt("commonmark", {"typographer": True})
+    parser.enable(["replacements", "smartquotes"])
+    parser.enable("table")
+    parser.use(mdit_py_plugins.anchors.anchors_plugin)
+    parser.use(mdit_py_plugins.footnote.footnote_plugin)
+    # Remove the starting hr from the footnote block. It's ugly and should be
+    # handled through CSS by putting a top border on section element.
+    parser.add_render_rule("footnote_block_open", _render_footnote_block_open)
+    html = parser.render(page["markdown"])
     return html
 
 
+def _render_footnote_block_open(self, tokens, idx, options, env):
+    """Render the footnote opening without the hr tag at the start."""
+    html = mdit_py_plugins.footnote.index.render_footnote_block_open(
+        self, tokens, idx, options, env
+    )
+    lines = html.split("\n")
+    if lines[0].strip().startswith("<hr"):
+        lines = lines[1:]
+    return "\n".join(lines)
+
+
 def render_markdown(page, config, site, build, jinja_env):
     """
     Render the templates in Markdown content of the page.

setup.cfg

@@ -11,7 +11,7 @@ license_file = LICENSE.txt
 platform = any
 keywords = html, markdown
 classifiers =
-    Development Status :: 3 - Alpha
+    Development Status :: 4 - Beta
     Intended Audience :: Developers
     License :: OSI Approved :: MIT License
     Natural Language :: English
@@ -36,7 +36,8 @@ zip_safe = True
 packages = find:
 python_requires = >=3.6
 install_requires =
-    myst-parser>=0.15.0
+    markdown-it-py>=2.0
+    mdit-py-plugins>=0.3
     jinja2>=3.0
     pyyaml>=5.4
     livereload>=2.6