pipenv: AttributeError: 'str' object has no attribute 'decode'

Continued from https://github.com/kennethreitz/pipenv/issues/541#issuecomment-329518287

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This issue is likely to show up on libraries that need to support both Python 2 and 3 where pipenv is being used to manage tooling. The feature:

Tell pip we’re using the required Python version, with trickery, for dependency resolution.

is keyed off python_version = "2.7", but to test that this project works with multiple versions of Python, my build matrix also runs with version of Python 3.

1. Install Python 2
2. Install Python 3 and set this as your global Python version
3. Install pipenv 7.0.2
4. Set a required Python version of 2.7 in Pipfile
5. Run $ pipenv install --python=python2

@kennethreitz here are some build logs demonstrating the issue: https://travis-ci.org/jacebrowning/verchew/builds/275563258