Sergeant

This is a major release fixing and improving many problems.
- Thread killers were replaced with one thread killer per as many threads as needed. It makes the threaded executor faster, slimmer, and simpler.
- Slave's process stderr is connected to the supervisor so it will have the chance to pipe its stderr in case of an unexpected exception in the slave's logic itself. The buffer was able to stall the slave's process because no process was cleaning its buffer. It is now configured to be unbuffered wishing it will fix the bug.
- There was a typo in threaded executor causing it to raise an exception after the completion of all the tasks.
- Requirements versions are now fixed
- Stop/Respawn can now be called in initialize/finalize

Breaking Changes:
- Worker config `executor` parameter was removed in favor of number_of_threads. See the documentation for how to choose the number.
- LogstashLogger was logging a "logger" field nested with name and level. These are now "name" and "level" in the parent document of the log.

- now every Redis connection holds a single connection resulted in a faster communication

- rename `consumable_only` to `include_delayed` to make this parameter clearer and more intuitive
- replace process killer from using udpserver to pipe - improved memory usage and more safe
- Redis connector performance improvements due to regressions from the previous version

- added Python 3.9 support!
- fix: supervisor took stderr from the slave denying it from displaying
its errors to the terminal

- supervisor now collects stderr from the slave
- supervisor now report on slaves that were interrupted by signals like
SIGINT
- fixed a bug when using the memory limiting feature of the supervisor
- slave's exception now propagated to the supervisor
- supervisor duplicate logs were merged
- supervisor tests were added
- mypy linting fixes
- isort fixes

- added support for delayed tasks