restore compatibility with python 3.7
add test case for unknown schema version error
add "eval" permission
improve loading of schema files
add tests for features from version 2 of source package JSON schema
incorporate version 2 of Hydrilla JSON schemas
make it easier to reuse/cache foreign package files when building a Hydrilla source package multiple times
always use APT packages for amd64 architecture
raise a more informative error when a file referenced from index.json is missing
properly handle percent-encoded characters in .deb file names
add missing english translations and verify message texts of raised exceptions in tests
support piggybacking on APT packages
use pyproject.toml for pytest configuration and move all tests to tests/ directory