Mirakuru

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Misc
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- Use strictier mypy checks

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Features
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- Replace `exp_sig` executor parameter with `expected_returncode`.
Parameter description already assumed that, however handing it assumed full
POSIX compatibility on the process side. Now the POSIX is only assumed if no
`expected_returncode` is passed to the executor, and returncode is simply that,
a returncode, nothing more

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Misc
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- Moved CI to Github Actions
- Blackified codebase
- Compacted Documentation into readme (was pretty small anyway)

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- [enhancement] Ability to set up expected exit code for executor. In Java exit codes 1- 127 have
special meaning, and the regular exit codes are offset by those of special meaning.

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- [enhancement] If process is being closed and the shutdown won't be clean (won't return exit code 0)
mirakuru will now rise ProcessFinishedWithError exception with exit_code

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- [bugfix][macos] Fixed typing issue on macOS