PyMC module doesn't install with '!pip install pymc'. The following error is shown:

Traceback (most recent call last):
File "/resources/common/.virtualenv/python2/bin/pip", line 7, in <module>
from pip.internal import main
File "/resources/common/.virtualenv/python2/lib/python2.7/site-packages/pip/internal/init.py", line 42, in <module>
from pip.internal import cmdoptions
File "/resources/common/.virtualenv/python2/lib/python2.7/site-packages/pip/internal/cmdoptions.py", line 16, in <module>
from pip.internal.index import (
File "/resources/common/.virtualenv/python2/lib/python2.7/site-packages/pip/internal/index.py", line 15, in <module>
from pip.vendor import html5lib, requests, six
File "/resources/common/.virtualenv/python2/lib/python2.7/site-packages/pip/vendor/requests/init.py", line 86, in <module>
from pip.vendor.urllib3.contrib import pyopenssl
File "/resources/common/.virtualenv/python2/lib/python2.7/site-packages/pip/vendor/urllib3/contrib/pyopenssl.py", line 46, in <module>
import OpenSSL.SSL
File "/usr/local/lib/python2.7/dist-packages/OpenSSL/init.py", line 8, in <module>
from OpenSSL import rand, crypto, SSL
File "/usr/local/lib/python2.7/dist-packages/OpenSSL/rand.py", line 11, in <module>
from OpenSSL.util import (
File "/usr/local/lib/python2.7/dist-packages/OpenSSL/util.py", line 3, in <module>
from cryptography.hazmat.bindings.openssl.binding import Binding
File "/usr/local/lib/python2.7/dist-packages/cryptography/hazmat/bindings/openssl/binding.py", line 13, in <module>
from cryptography.hazmat.bindings.openssl import ffi, lib
AttributeError: 'module' object has no attribute 'initcffi10external_module'