All the Metadata You Could Ever Want

Major config changes around logging in, especially for those with an API_CLIENT_ID, please consult the README Advanced API Options!
Completely rewrite API response -> object metadata -> file tagging pipeline, including bulk-fetching for non-DevAPI users
Create a metadata import/export system (IMPORT_ZMD/EXPORT_ZMD) with its own human-readable, self-referential format (.zmd)
Expanded --verify-library (-v) scope and improved performance
Modularize .song_archive/.song_id historians (running --verify-library to update recommended)

New Features

Added test/dry run mode (-T, --test, --dry-run)
Add SKIP_BY_ISRC config to skip tracks with duplicate International Standard Recording Codes for [Feature Request] #197
Add FETCH_DELAY and ESCALATING_DELAY configs for [Feature Request] #217

Completely rewrite the login flow, split Login5 credentials and DevAPI credentials into separate files
Dozens of bug fixes, especially for incomplete/interrupted API metadata responses
adds 429 error and "retry-after" header from response to logs by @DerKO9 in #212
gets display_name from librespot by @DerKO9 in #213
fix: alias platform.system to stop shadowing os.system in termoutput by @SAY-5 in #220
guard against duration_ms=None at fetch_content_stream rate-limiter by @abhishek1337chatterjee in #221

Full Changelog: v0.17.0...v0.21.0