v0.7.5: Cleaner, more efficient code & better documentation

This release adds the User, Network, Terminal, MessageBus & Error classes to the package. They greatly improve the readability, usability, expressiveness & concern separation of the codebase. The resulting code is heavily refactored & much cleaner.

The aiofiles dependency was also removed since GnuPG in general doesn't play well with asynchronous or multi-threaded code. The asynchronous networking code will, however, stay with the package.