Clean up build and code

[roskakori]

Goals:

• [x] The repository has a pre-commit hook with.
  • [x] black
  • [x] isort
  • [x] various helpful standard checks
• [x] The integration with the now defunct landscape.io is removed.
• [x] ant is removed and the remaining few targets are mapped to shell scripts with set -e as first line.
• [x] The copyright year is bumped to 2021.
• [x] Obsolete requirements files are removed.