Fails on too old pip:

Downloading/unpacking cryptography>=2.1.4 (from
pyOpenSSL->autoca==0.3.2)
  Running setup.py (path:/tmp/pip-build-ZJGhFW/cryptography/setup.py) egg_info for package cryptography
    error in cryptography setup command: Invalid environment marker: platform_python_implementation != 'PyPy'
    Complete output from command python setup.py egg_info:
    error in cryptography setup command: Invalid environment marker: platform_python_implementation != 'PyPy'