sysutils/py-plumbum: update to 1.6.7

• PyPI sources do not ship LICENSE_FILE or tests
• ChangeLog:
  • Commands: Added run_* methods as an alternative to modifiers
  • CLI: Added support for ALLOW_ABREV
  • CLI: Added DESCRIPTION_MORE, preserves spacing
  • Color: Avoid throwing error in atexit in special cases (like pytest)
  • Smaller bugfixes and other testing improvements.