Replace posix-ipc by the shm from Python standard library

[stephane-caron]

A blocker to this proposal is that multiprocessing.shared_memory does not provide access to the underlying file descriptor in its API.

We could try accessing the internal _fd though... :thinking:

[coveralls]

Pull Request Test Coverage Report for Build 7842113818

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.466%

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Totals
Change from base Build 7804297721:	0.0%
Covered Lines:	1758
Relevant Lines:	1965

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💛 - Coveralls

[stephane-caron]

No noticeable impact on performance :+1: