Major update (2021/08/26)

Changes since 0.5.0

1. Added unit testing
2. Added checking for argument order_by in User.lists
3. Added username availability checking API
4. Added delete media uploaded by a user API
5. Added a new parameter called "external_ids" to User.create_modify
6. Updated dependency httpx to 0.18.2
7. Implemented a simple MIME type guessing based on magic
8. Support sending media in an announcement
9. Management.announce is now a helper method for old Management.announce and Management.announce_all
10. Some PEP8