cannot import tf_conversions in Python

[ninafiona]

Ubuntu 18.04, ros melodic, Python 2.7

I tried to import tf_conversions, but got the following error:

import tf_conversions
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/ros/melodic/lib/python2.7/dist-packages/tf_conversions/__init__.py", line 30, in <module>
    from .posemath import *
  File "/opt/ros/melodic/lib/python2.7/dist-packages/tf_conversions/posemath.py", line 34, in <module>
    from PyKDL import *
ImportError: No module named sip

I tried to build orocos-kdl with python-orocos-kdl from source, but it doesn't work if I also have another version installed from ros-orocos-kdl (I can only build and install when ros-orocos-kdl isn't installed).

Similar issue here: orocos/orocos_kinematics_dynamics#287