Yesterday I had this issue, I cloned letsencrypt from GitHub and tried running it:

Code:
Collecting certbot-nginx==0.30.0 (from -r /tmp/tmp.NZM1FU6no3/letsencrypt-auto-requirements.txt (line 218))
  Downloading https://files.pythonhosted.org/packages/ad/15/7f42f31ba068f837816647135a0704a8c89aad685df160407e92870c0277/certbot_nginx-0.30.0-py2.py3-none-any.whl (90kB)
Requirement already satisfied: setuptools>=1.0 in /opt/eff.org/certbot/venv/lib/python2.7/site-packages (from josepy==1.1.0->-r /tmp/tmp.NZM1FU6no3/letsencrypt-auto-requirements.txt (line 95)) (40.6.3)
Exception:
Traceback (most recent call last):
  File "/opt/eff.org/certbot/venv/local/lib/python2.7/site-packages/pip/_internal/cli/base_command.py", line 176, in main
    status = self.run(options, args)
  File "/opt/eff.org/certbot/venv/local/lib/python2.7/site-packages/pip/_internal/commands/install.py", line 346, in run
    session=session, autobuilding=True
  File "/opt/eff.org/certbot/venv/local/lib/python2.7/site-packages/pip/_internal/wheel.py", line 848, in build
    assert building_is_possible
AssertionError