Bump version: 0.5.1 → 0.5.2

What's Changed

• Improve code quality: simplify is_leap_year and refactor ISO 8601 parsing by @chetmancini in #19
• Fix age_in_years leap year birthday edge case by @chetmancini in #20
• Improve code quality: accept Iterable for holidays and add edge case tests by @chetmancini in #21

Full Changelog: v0.5.1...v0.5.2