Python 3.4 HTMLParser constructor has a convert_charrefs argument, which Python 3.5 warns about

The Python 3.4 convert_charrefs argument is missing from the Python 3.3 backport

https://github.com/PythonCharmers/python-future/blob/master/src/future/backports/html/parser.py#L122

Python 3.5 issues a warning if it is not used.

```
DeprecationWarning: The value of convert_charrefs will become True in 3.5. You are encouraged to set the value explicitly.
```
