Incompatible with pip 18.1

[Midnighter]

The newest pip release 18.1 finally moved their API around and pipdeptree no longer works.

Traceback (most recent call last):
  File "/home/moritz/.virtualenvs/test-cobra/lib/python3.6/site-packages/pipdeptree.py", line 17, in <module>
    from pip._internal import get_installed_distributions
ImportError: cannot import name 'get_installed_distributions'

Somehow I was under the impression that pipdeptree was not using the internal pip API but that's obviously wrong. Importing from a module marked as _internal seems... less than ideal 😬

[zachliu]

I hereby confirm this incompatibility 😂