Do not use setsid directly, use dedicated start_new_session

[yarikoptic]

Just ran into
https://stackoverflow.com/questions/42257512/difference-between-subprocess-popen-preexec-fn-and-start-new-session-in-python which points to python docs at
https://docs.python.org/3.4/library/subprocess.html?highlight=subprocess#popen-constructor

Warning The preexec_fn parameter is not safe to use in the presence
of threads in your application. The child process could deadlock before
exec is called. If you must use it, keep it trivial! Minimize the number of
libraries you call into.

Note If you need to modify the environment for the child use the env
parameter rather than doing it in a preexec_fn. The start_new_session
parameter can take the place of a previously common use of preexec_fn to
call os.setsid() in the child.

So smells like we better use this dedicated option added in 3.2 .

But may be it was considered and argued against @asmacdo and doesn't work as desired

(there is a separate discussion on possibly using group not session ids in #82)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.56%. Comparing base (ebf7b17) to head (7f78031).
Report is 7 commits behind head on main.

Additional details and impacted files

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##             main     #155   +/-   ##
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  Coverage   95.56%   95.56%           
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  Files           2        2           
  Lines         451      451           
  Branches       69       69           
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  Hits          431      431           
  Misses         10       10           
  Partials       10       10           

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[asmacdo]

I don't think this was considered, looks like an improvement to me. I'm curious if start_new_session is actually slower, or if its just random flake.

For the failing test I started that requirement as tight as possible, opting to relaxed that requirement only as needed, which I've done in latest push.