Pipfile failing for valid dependencies (six 1.12.0) - pipenv

Question

A valid Pipfile which builds on some machines isn't building on others, and dies with a dependency error.
We see this error when running pipenv install --dev even though it looks like six version 1.12.0 should match the set of constraints:
ERROR: ERROR: Could not find a version that matches six>=1.10.0,>=1.12,>=1.2,>=1.5.2,>=1.6.1,>=1.7.0,>=1.9.0,~=1.10.0
Tried: 0.9.0, 0.9.1, 0.9.2, 1.0.0, 1.1.0, 1.2.0, 1.3.0, 1.4.0, 1.4.1, 1.5.0, 1.5.0, 1.5.1, 1.5.1, 1.5.2, 1.5.2, 1.6.0, 1.6.0, 1.6.1, 1.6.1, 1.7.0, 1.7.0, 1.7.1, 1.7.1, 1.7.2, 1.7.2, 1.7.3, 1.7.3, 1.8.0, 1.8.0, 1.9.0, 1.9.0, 1.10.0, 1.10.0, 1.11.0, 1.11.0, 1.12.0, 1.12.0
S
The original Pipfile is:
[[source]]
url = "https://pypi.python.org/simple"
verify_ssl = true
[dev-packages]
pytest = "*"
ipdb = "*"
django-environ = "*"
responses = "*"
django-extensions = "*"
django = "<=1.11.7"
pytest-cov = "*"
pytest-django = "*"
"flake8" = "*"
pytest-xdist = "*"
[packages]
grpcio = "*"
pytest = "*"
requests = "*"
pytest-django = "*"
"psycopg2" = "*"
arrow = "*"
retrying = "*"
djangorestframework = "*"
"autopep8" = "*"
dj-environ = "*"
pytz = "*"
pillow = "*"
raven = "*"
colorlog = "*"
django-extensions = "*"
redis = "<3.0.0"
rq = "<0.11.0"
keen = "*"
pymongo = "*"
django-simple-history = "*"
python-stdnum = "*"
dictdiffer = "*"
shippo = "*"
shopifyapi = "*"
mypy = "*"
boto = "*"
google-cloud-vision = "*"
twilio = "*"
django-phonenumber-field = "*"
python-bcrypt = "*"
phonenumbers = "*"
core = {editable = true,path = "."}
"tinys3" = "*"
dialogflow = "*"
python-twitter = "*"
sentry-sdk = "*"
"oauth2client" = "*"
tenacity = "*"
fakeredis = "*"
[requires]
python_version = "3.6"
We've tried a number of options, such as pipenv install --ignore-pipfile to use a Pipfile.lock from a machine that appeared to work, but it didn't install anything. Running pipenv graph didn't produce any output. Running pipenv install --skip-lock failed with the original error.