Make theses/models.py PEP8 compliant

I've opted for hanging indentation

foo = long_function_name(
    var_one, var_two,
    var_three, var_four)

instead of aligning with opening delimiter

foo = long_function_name(var_one, var_two,
                         var_three, var_four)

because the former makes more sense to me. See
https://www.python.org/dev/peps/pep-0008/#indentation