mitogen.parent.Connection._first_stage() does not import platform

[moreati]

In the first stage there is a workaround for certain MacOS versions. However it uses platform.release(), without any corresponding import in the output ofget_boot_command(). I believe this would cause an error, with a matching MacOS target.

https://github.com/dw/mitogen/blob/cc8f9a016965876bcd9ec390d53035d6ed842b07/mitogen/parent.py#L1438-L1442 Is this workaround still needed? If so we could probably use os.uname() instead of platform.release().

[moreati]

sys.version[:3] will fail on Python 3.10

[moreati]

Some experimentation to reduce _first_stage(), so far saved 200ish bytes https://github.com/dw/mitogen/compare/master...moreati:first-stage-golf