[core-workflow] Updated draft of PEP 512

[francismb]

Hi Brett,
> 
> Change sys._mercurial
> '''''''''''''''''''''
> Once Python is no longer kept in Mercurial, the ``sys._mercurial``
> attribute will need to be removed. An equivalent ``sys._git``
> attribute will be needed to take its place.
> 

Would it make sense to change it to something more general like:

sys._version_control_system or
sys._vcs  with maybe a new field for the type of vcs: 'mercurial',
'git' or 'the next one' :-)


Regards,
francis