pysbol: Segmentation fault after PartShop pull on Linux

[tim-tx]

Steps to reproduce:

1. Run the following snippet (the parts.xml referenced below is here):

   from sbol import *
   
   namespace = "http://my_namespace.org"
   setHomespace(namespace)
   doc = Document()
   generic_parts = Document('parts.xml')
   generic_parts.copy("http://examples.org", doc)
   partshop = PartShop('https://synbiohub.org/public/igem')
   records = partshop.search('interlab')
   medium_device_uri = records[0].identity
   

2. Run the following line

   partshop.pull(medium_device_uri, doc)
   

Result:

[1]    6109 segmentation fault (core dumped)  ipython

Tested on python 3.6 on Arch Linux, in ipython and regular python shell.