Gelf-formatter

- fix: stringify additional attributes (16) (18)
- chore: remove support for python 2.7 and 3.5, add 3.9 (19)

Thanks to 0mbi for their contribution! 🎉

- fix: set asctime if required (5)
- feat: selectively ignore fields at the root level (1)
- chore: add tests for py38 (3)

Thanks to girstenbrei and ozooner for their contribution! 🎉

🎉 First release!