--process-dependency-links is removed in pip 19.0

[tnguyen14]

process_dependency_links option is used in https://github.com/poise/poise-python/blob/master/lib/poise_python/resources/python_package.rb#L73, and it's removed in pip 19.0, so using python_package is giving us an error:

STDERR: Traceback (most recent call last):
  File "<stdin>", line 33, in <module>
AttributeError: Values instance has no attribute 'process_dependency_links'

[wmantly]

@tnguyen14 Do you have a workaround for this?

[signe]

Dupe of #140