Use tempfile module in g2p

This is really a small PR.

- use the `tempfile` module instead of a hardcoded temporary file in the main jasper directory.
- use `with` statement instead of `f = open(...) ... f.close()`
I feel like this is a particularly good case to do:

```py
with tempfile.NamedTemporaryFile(suffix='.g2p', delete=False) as f:
    f.write(full_text)
    output = translateFile(f.name)
return output
```