Update to 3.20180228. From the changelog:

* core: Don't send relative redirect URLs when behind a reverse proxy
* core: Escape backticks etc. in directive error messages as HTML
  entities so that the error message is not subsequently parsed as
  Markdown
* mdwn: Enable fenced code blocks, PHP Markdown Extra-style definition
  lists and GitHub-style extensions to HTML tag syntax when used with
  Discount >= 2.2.0 (Closes: #[888055])
* img: Fix auto-detection of image format (if enabled, which is
  strongly discouraged) with ImageMagick >= 6.9.8-3
* rst: Use Python 3 instead of Python 2
* build: `set -e` before each `for` loop, so that errors are reliably
  trapped
* build: Use if/then instead of `||` so that the `-e` flag works
* build: Ensure that pm_to_blib finishes before rewriting shebang lines
* t: Make the img test pass with ImageMagick >= 6.9.8-3
  (Closes: #[891647])
* debian: Remove unused Lintian overrides for duplicate word false positives
* debian: Declare compliance with Debian Policy 4.1.3

(schmonz)