Tweaked by tabbott to not remove it from lister.py, linter_lib, and
friends, since those are intended to support both Python 2 and 3
(we're planning to extract them from the repository).
This code is not directly related to the template parser, so it
can safely live in its own file.
The only significant change to the code is to the signature of
`html_branches` so that it can be called without requiring a file.
Since it's only used in html_grep, that has been updated to reflect
this change.
Fixes: #1774.
