Pipenv: Dependencies could not be resolves when installing pytest with pytest-cov

Issue description

I cannot install pytest and pytest-cov as it complains about dependencies not being resolved.

Steps to replicate and output

My Pipfile

[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"

[packages]
pytest = "*"
pytest-cov = "*"

[dev-packages]

[requires]
python_version = "3.6"

 $ pipenv install
Pipfile.lock (ca72e7) out of date, updating to (4ccbf7)...
Locking [dev-packages] dependencies...
Locking [packages] dependencies...

Warning: Your dependencies could not be resolved. You likely have a mismatch in your sub-dependencies.
  You can use $ pipenv install --skip-lock to bypass this mechanism, then run $ pipenv graph to inspect the situation.
  Hint: try $ pipenv lock --pre if it is a pre-release dependency.
Could not find a version that matches pluggy<0.7,>=0.5,>=0.7
Tried: 0.3.0, 0.3.0, 0.3.1, 0.3.1, 0.4.0, 0.4.0, 0.5.0, 0.5.1, 0.5.1, 0.5.2, 0.5.2, 0.6.0, 0.6.0, 0.6.0, 0.7.1, 0.7.1
There are incompatible versions in the resolved dependencies.

I then do a:

$ pipenv install --skip-lock
$ pipenv graph
pytest-cov==2.5.1
  - coverage [required: >=3.7.1, installed: 4.5.1]
  - pytest [required: >=2.6.0, installed: 3.7.0]
    - atomicwrites [required: >=1.0, installed: 1.1.5]
    - attrs [required: >=17.4.0, installed: 18.1.0]
    - more-itertools [required: >=4.0.0, installed: 4.3.0]
      - six [required: >=1.0.0,<2.0.0, installed: 1.11.0]
    - pluggy [required: >=0.7, installed: 0.7.1]
    - py [required: >=1.5.0, installed: 1.5.4]
    - setuptools [required: Any, installed: 40.0.0]
    - six [required: >=1.10.0, installed: 1.11.0]

Dependencies look fine.