Update release

Important notes

• The support of Python 3.6 is dropped from this version.

What's Changed

• Require init.py in sub-packages by @Hoshock in #20
• Update development environment by @astropenguin in #27
• Update supported Python versions by @astropenguin in #29
• Release v0.6.0 by @astropenguin in #31

New Contributors

• @Hoshock made their first contribution in #20

Full Changelog: v0.5.9...v0.6.0