pip download module has been moved to pip._internal

[xiangninglyu]

Updating the pippuppet.py script to use the correct module with:

from pip._internal.download import PipSession

will still result in error because the sprinter script seems to be manually overriding any changes to the file:

File "/private/tmp/sprinter-sandbox-xiangningl/sprinter/external/pippuppet.py", line 6, in <module>
    from pip.download import PipSession
ImportError: No module named download