init: zstd is already python3-ready, so don't run it through our importer

config: guard against setconfig specifying unicode values on py3 This was leading to some difficult to trace problems because the values were set in one place, but then blew up much later in the program. Exploding violently with an assertion seems reasonable here.

dispatch: enforce bytes when converting boolean flags to config items This fixes --verbose on Python 3.

py3: add a test that proves %include and missing-extension code works