ci: fix for macos builds

setup.py: license classifier is deprecated, removing

use c++17

version bump

Allow passing docid as string instead of bytes #26

For backward-compatibility, passing bytes is still allowed as well

updated dependencies for frog 0.31

macosx wheel building: compile against homebrew's 'keg-only' libxml2 …

…and icu4c

version bump for compilation against frog v0.30

metadata update

version bump for compilation against ucto v0.28