get-pip.py fails when run with pip==9.0.* version specification.

[jwalgran]

TASK [azavea.pip : Install pip] ************************************************
fatal: [default]: FAILED! => {"changed": true, "cmd": ["python", "get-pip.py", "pip==9.0.*"], "delta": "0:00:00.915082", "end": "2018-10-05 17:06:15.290511", "failed": true, "msg": "non-zero return code", "rc": 1, "start": "2018-10-05 17:06:14.375429", "stderr": "Double requirement given: pip (already in pip==9.0.*, name='pip')", "stderr_lines": ["Double requirement given: pip (already in pip==9.0.*, name='pip')"], "stdout": "", "stdout_lines": []}

This currently unanswered SO question looks to be related
https://stackoverflow.com/questions/52669471/how-to-install-specific-pip-version-with-pip-18-1

[hectcastro]

See also: pypa/get-pip#37

[hectcastro]

For this task, we should try to add an additional variable so that a get-pip.py version can be supplied. The goal being to target get-pip.py under this subdirectory: https://bootstrap.pypa.io/3.3/