Python 3 compatibility fix

[This](https://github.com/NFicano/python-youtube-download/commit/f9849241120e9bec548f8d7113409d2be6291a0c) commit broke compatibility with Python 3. In line  41 `self._buffer` is set to `buffer` which doesn't exists in 3.X.

The equivalent would be `memoryview` which works in 2.7+ but seeing as `buffer` was not used at all, (It was overridden two lines below) I just removed it.