Changes to sys.ps1 and sys.ps2 don't update the interpreter prompts

So, for example, the following doesn't work, either in the PYTHONSTARTUP file or once the interpreter is running:

``` python
import sys
sys.ps1 = '> '
sys.ps2 = '. '
```

---
- Bitbucket: https://bitbucket.org/bobf/bpython/issue/267
- Originally Reported By: [Michael Schuller](http://bitbucket.org/_petronius)
- Originally Created At: 2014-03-10T13:50:01.042
