⬆️ Comply with Commonmark 0.31.2

[chrisjsewell]

This PR ports markdown-it/markdown-it@cd24778, which in turn complies with https://spec.commonmark.org/0.31.2/changes.html:

• Unicode:

  -  an [ASCII punctuation character] or anything in
  - he general Unicode categories  `Pc`, `Pd`, `Pe`, `Pf`, `Pi`, `Po`, or `Ps`.
  + a character in the Unicode `P` (puncuation) or `S` (symbol) general categories. 

• HTML comments:

  - An HTML comment consists of `<!--` + *text* + `-->`,
  - where *text* does not start with `>` or `->`, does not end with `-`, and does not contain `--`.
  - (See the [HTML5 spec](http://www.w3.org/TR/html5/syntax.html#comments).)
  + An [HTML comment](@) consists of `<!-->`, `<!--->`, or  `<!--`, a string of characters not including the string `-->`, and `-->`
  + (see the [HTML spec](https://html.spec.whatwg.org/multipage/parsing.html#markup-declaration-open-state)). 

• HTML blocks:

  - `section`, `source`, `summary`, `table`, `tbody`, `td`,
  + `search`, `section`, `summary`, `table`, `tbody`, `td`, 

Another notable change in the diff was Setext headers, although there appears to be no change to the port of tests in this respect:

- If a line containing a single `-` can be interpreted as an
- empty [list items], it should be interpreted this way
- and not as a [setext heading underline]. 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.77%. Comparing base (8eb20ac) to head (7de9006).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #362   +/-   ##
=======================================
  Coverage   95.77%   95.77%           
=======================================
  Files          64       64           
  Lines        3429     3429           
=======================================
  Hits         3284     3284           
  Misses        145      145           

Flag	Coverage Δ
pytests	95.77% <100.00%> (ø)

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