CuVec v2.12.0 stable

• add SWIG retarray convenience function (#30)
• misc build, test & CI framework updates (#30)
  • migrate setup.cfg => pyproject.toml
  • drop redundant Python ROOT_DIR
  • only find Python Development.Module
  • add swig to mandatory dependencies
  • bump pre-commit
• minor code tidy
  • drop redundant typecast
• update docs & tests