provide markdown extension to render graphviz

this is server side, and a more standard format, and thus I like it more
than mermaid, which I've been using at work. but, I really wanted a
server-side option (see my manifesto) for drawing relationship graphs,
for D&D stuff of all things.

this adds an optional 'graphviz' feature to package installation which
consequently depends on pydot

incorporealcms/mdx/__init__.py

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+"""Markdown extensions."""

incorporealcms/mdx/pydot.py

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+"""Serve dot diagrams inline."""
+import base64
+import re
+
+import markdown
+import pydot
+
+
+class InlinePydot(markdown.Extension):
+    """Wrap the markdown prepcoressor."""
+
+    def extendMarkdown(self, md, md_globals):
+        """Add InlinePydotPreprocessor to the Markdown instance."""
+        md.registerExtension(self)
+        md.preprocessors.add('dot_block', InlinePydotPreprocessor(md), '_begin')
+
+
+class InlinePydotPreprocessor(markdown.preprocessors.Preprocessor):
+    """Identify dot codeblocks and run them through pydot."""
+
+    BLOCK_RE = re.compile(r'~~~pydot:(?P<filename>[^\s]+)\n(?P<content>.*?)~~~', re.DOTALL)
+
+    def run(self, lines):
+        """Match and generate diagrams from dot code blocks."""
+        text = '\n'.join(lines)
+        for match in self.BLOCK_RE.finditer(text):
+            filename = match.group(1)
+            dot_string = match.group(2)
+
+            # use pydot to turn the text into pydot
+            graphs = pydot.graph_from_dot_data(dot_string)
+
+            # encode the image and provide as an inline image in markdown
+            encoded_image = base64.b64encode(graphs[0].create_png()).decode('ascii')
+            data_path = f'data:image/png;base64,{encoded_image}'
+            inline_image = f'![{filename}]({data_path})'
+
+            # replace the image in the output markdown
+            text = f'{text[:match.start()]}\n{inline_image}\n{text[match.end():]}'
+
+        return text.split('\n')
+
+
+def makeExtension(*args, **kwargs):
+    """Provide the extension to the markdown extension loader."""
+    return InlinePydot(*args, **kwargs)