- update to 2.5.1:
* [pull #590] Fix underscores within bold text getting
emphasized
* [pull #591] Add Alerts extra
* [pull #595] Fix img alt text being processed as markdown
* [pull #598] Add `link-shortrefs` extra
* [pull #600] Use urandom for SECRET_SALT
* [pull #602] Fix XSS issue in safe mode
* [pull #604] Fix XSS injection in image URLs
* [pull #519] Add support for custom extras
* [pull #519] Drop Python 3.5 support
* [pull #568] Add `prepend` arg to toc extra
* [pull #569] Process HTML comments as markdown in 'escape'
safe mode
* [pull #570] Fix syntax warnings in test suite
* [pull #572] Process inline tags as HTML blocks when they span
multiple lines
* [pull #573] Add new LaTeX Extra
* [pull #576] Fix `html`, `head` and `body` tags being wrapped
in `

` tags
* [pull #578] Graceful handling of broken lists when cuddled-
lists extra is enabled
* [pull #581] Add type hints
* [pull #581] Drop Python 3.6 and 3.7 support
* [pull #582] Fix fenced code blocks breaking lists
* [pull #586] Fix #583 by tweaking incomplete tag regex
* [pull #587] Fix AssertionError on malformed HTML