Fix locale parameter, add fallback_to_build_date option

Big thanks to @Guts for all his work on this release!

Fixed

• locale did not change date format output #21

Added

• New option fallback_to_build_date that uses the build date instead of a git date. Useful when git is not available in build environment #20

Improved

• black-ened code
• improved and extended unit tests
• codebase:
  • include logging.debug() statements
  • add some type hinting, improve import orders etc