Fix utils.py for PyPy on Windows

The line
```python
from __future__ import unicode_literals
```
introduced in commit [ecc0c5ee01f0e5bdd6af0c32cb5b4adcb2a2f78c](https://github.com/rg3/youtube-dl/commit/ecc0c5ee01f0e5bdd6af0c32cb5b4adcb2a2f78c) broke youtube-dl for PyPy on Windows, making it unable to locate WinAPI functions.
Error: "TypeError: function name must be a string or integer"

Adding "b" prefix to strings with WinAPI function names fixes it.