yamldcop.py incompatible with YAML library?

[rpgoldman]

After tweaking my issue with the version of PuLP (#37), I tried the test case again, and this time I get an error in yamldcop.py:

$ pydcop solve --algo dpop graph-coloring.yaml
Traceback (most recent call last):
  File "/Users/rpg/.virtualenvs/pydcop/bin/dcop.py", line 181, in <module>
    main()
  File "/Users/rpg/.virtualenvs/pydcop/bin/dcop.py", line 112, in main
    args.func(args)
  File "/Users/rpg/.virtualenvs/pydcop/lib/python3.9/site-packages/pydcop/commands/solve.py", line 254, in run_cmd
    dcop = load_dcop_from_file(args.dcop_files)
  File "/Users/rpg/.virtualenvs/pydcop/lib/python3.9/site-packages/pydcop/dcop/yamldcop.py", line 80, in load_dcop_from_file
    return load_dcop(content)
  File "/Users/rpg/.virtualenvs/pydcop/lib/python3.9/site-packages/pydcop/dcop/yamldcop.py", line 84, in load_dcop
    loaded = yaml.load(dcop_str)
TypeError: load() missing 1 required positional argument: 'Loader'

There's no version specification for pyyaml, so that's likely the problem here. Looking at the pyyaml changelog, we see that the load function was changed to require a Loader with version 6.0

Parsing does better when I downgrade to version 5.4.1, so that's probably a good idea.