tuf v0.12.0

• Add backwards incompatible TUF spec version checks (#842, #844, #854, #914)
• Adopt securesystemslib v0.12.0 update (#909, #910, #855, #912, #934)
• Fix multi-root rotation (#885, #930)
• Fix duplicate schema definitions (#929)
• Refactor metadata generation (#836)
• Refactor securesystemslib interface (#919)
• Update implementation roadmap (#833)
• Improve tests and testing infrastructure (#825, #839, #890, #915, #892, #923)
• Improve documentation (#824, #849, #852, #853, #893, #924, #928, et al.)
• Update misc dependencies (#850, #851, #916, #922, #926, #931)