ValueError: unsupported pickle protocol: 5

Hi. I am trying to load 'london_transport' and 'eu_airlines' networks with graph-tool 2.35 and Python 3.6.9. Both loading the uncompressed file and using gt.collection.ns throw 'ValueError: unsupported pickle protocol: 5'; as shown below:

from graph_tool.all import *
g = collection.ns['london_transport']

Traceback (most recent call last):

  File "<ipython-input-6-e0b0e8c60494>", line 1, in <module>
    g = load_graph(out_dir+'/london_transport.gt')

  File "/usr/lib/python3/dist-packages/graph_tool/__init__.py", line 3346, in load_graph
    g.load(file_name, fmt, ignore_vp, ignore_ep, ignore_gp)

  File "/usr/lib/python3/dist-packages/graph_tool/__init__.py", line 2889, in load
    ignore_ep, ignore_gp)

  File "/usr/lib/python3/dist-packages/graph_tool/gt_io.py", line 71, in unpickler
    return pickle.load(sstream, encoding="bytes")

ValueError: unsupported pickle protocol: 5