A new root and variant python config field is provided with two new
fields below, version and requirements.
The former, version, should specify the Python executable to use when
executing related package installation commands and ostensibly the same
executable that will be used to run the application.
The latter, requirements, should specify all pip requirements files
such that a compiler that supports layered filesystems (e.g. Docker) can
output separate instructions that will invalidate cache layers for
changes to those files independently of changes to the rest of the
Python related instructions will be generated only if either version
or requirements are given.