ResourceWarning on tests

Not seen this locally, oddly, but Travis CI always hits it:

```
Exception ResourceWarning: ResourceWarning("unclosed file <_io.BufferedReader name='/home/travis/build/html5lib/html5lib-python/html5lib/tests/testdata/encoding/chardet/test_big5.txt'>",) in <_io.FileIO name='/home/travis/build/html5lib/html5lib-python/html5lib/tests/testdata/encoding/chardet/test_big5.txt' mode='rb'> ignored
```
