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