In HTML, the line break immediately following a start tag is ignored
(see: https://www.w3.org/TR/html4/appendix/notes.html#h-B.3.1). An
extra span tag has been introduced in the upstream Pygments
HtmlFormatter in order to preserve the first new line. The Bugdown
Tests as well as our fenced_code.js frontend markdown processor have
been updated to reflect this new behavior.
See #2357. We now support `~~~ .py ` with that trailing space.
Note that the test coverage is Python-side only due to
bugdown_matches_marked being set to false, since we don't yet
support language syntax on the client side.
(imported from commit ccd5fcb0eee01478d349161400103480678d7486)