TypeError when loading config from --config-file CLI option

[tbcvl]

To reproduce, call septentrion CLI with --config-file option, e.g.:
septentrion --config-file=some_file.ini --target-version=1.0 migrate

It will fail:

  ...
  File "/tmp/venv/lib/python3.7/site-packages/septentrion/cli.py", line 37, in load_config
    ctx.default_map = configuration.load_configuration_files(value)
  File "/tmp/venv/lib/python3.7/site-packages/septentrion/configuration.py", line 193, in load_configuration_files
    return parse_configuration_file(file_contents)
  File "/tmp/venv/lib/python3.7/site-packages/septentrion/configuration.py", line 82, in parse_configuration_file
    parser.read_string(content)
  File "/usr/local/lib/python3.7/configparser.py", line 721, in read_string
    sfile = io.StringIO(string)
TypeError: initial_value must be str or None, not bytes
script returned exit code 1