Shellous

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- [API] In audit_callback, signal names no longer have prefix "Signals."; just the signal name, e.g. "SIGTERM".
- [LATERAL] Run CI tests on Python 3.11-dev.
- [LATERAL] Update copyright year.

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- [FEATURE] Add the `.result` modifier to Command and Pipeline.
- [BUGFIX] Fix failing test on Python 3.10.1 due to change in behavior of `asyncio.wait_for`.
- [LATERAL] Add more timeout-cancellation tests.

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- [BUGFIX] Fix race condition in process substitution where process is left running due to exception.
- [BUGFIX] Handle pidfd_open case where we run out of file descriptors.

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- [FEATURE] Custom child watcher class, DefaultChildWatcher, automatically picks between pidfd, kqueue and thread implementation.
- [BUGFIX] Manually reap process when child watcher fails to detect child exit. (178)
- [LATERAL] Add tests to make sure that Command, Pipeline and Result objects can be pickled.
- [LATERAL] Remove context reference from command Options class.

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- [FEATURE] Experimental child watcher now supports linux using pidfd.
- [BUGFIX] Fix process still running error (172).
- [BUGFIX] Improve exit status reporting after a `SIGTERM` signal with experimental child watcher.
- [LATERAL] Add continuous integration support for Alpine linux.

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- [FEATURE] Add % operator to concatenate commands, i.e. sh("nohup") % sh("echo") == sh("nohup", "echo")
- [API] Replace command and pipe's ~ operator with .writable property.
- [BUGFIX] Fix bug where GeneratorExit was not being re-raised.
- [BUGFIX] Clean captured output in tests (151).
- [LATERAL] Add tests to check for open/inherited file descriptors in subprocess using lsof.