Pychroot

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- Fix build with newer versions of snakeoil.

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- Add py3.9 support.

- Fix pychroot script hang on >=py3.7 (30).

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- Add py3.8 support.

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- Minimum supported python version is now python3.6 (python2 support dropped).

- Fix handling of environment variable mounts to nonexistent paths.

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- Return the exit status of the child process instead of 0 for the pychroot
script. This makes pychroot more compatible with chroot's behavior.

- Run clean up before exit after receiving SIGINT/SIGTERM, previously stub
files/directories that were created for bind mounts weren't properly cleaned
up in the chroot after the parent process received these signals and exited.

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- Handle single file bind mount creation failures.