Replace vendored markdown2 w/ official upstream (#836)

* Replace vendored markdown2 w/ official upstream

Closes #815.

All tests seem to pass, so this seems safe?

* [autofix.ci] apply automated fixes

* delete + ignore missing imports

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: jinnovation

CHANGELOG.md

@@ -6,6 +6,8 @@
 
 - Fix linking of identifiers that contain unicode characters.
   ([#831](https://github.com/mitmproxy/pdoc/issues/831), @iFreilicht)
+- Replace vendored version of `markdown2` with the [official
+  upstream](https://github.com/trentm/python-markdown2)
 
 ## 2025-06-04: pdoc 15.0.4