Bellybutton breaks on non-ascii characters in source

[hchasestevens]

Traceback:

Traceback (most recent call last):
  File "/usr/local/bin/bellybutton", line 10, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.6/dist-packages/bellybutton/cli.py", line 218, in main
    for arg in args.func.__code__.co_varnames[:args.func.__code__.co_argcount]
  File "/usr/local/lib/python3.6/dist-packages/bellybutton/cli.py", line 191, in lint
    for failure in linting_failures(filepaths, rules):
  File "/usr/local/lib/python3.6/dist-packages/bellybutton/cli.py", line 134, in linting_failures
    for filepath, file_contents in files:
  File "/usr/local/lib/python3.6/dist-packages/bellybutton/cli.py", line 109, in open_python_files
    contents = f.read()
  File "/usr/lib/python3.6/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe3 in position 3487: ordinal not in range(128)