Use different method for getting ext_suffix

```python
ext_suffix = get_config_var("EXT_SUFFIX") or get_config_var("SO")
```

because `get_config_var("SO")` returns None in Python 3.4.0a4 because the `"SO"` variable is deprecated and `"EXT_SUFFIX"` is the new way to get this information (see: http://bugs.python.org/issue19555)

This fixes `TypeError: Can't convert 'NoneType' object to str implicitly` errors when running the tests on Python 3.4.0a4.

Note that the fact that I had to make this change in 3 places suggests that it would be nice to refactor so that this code lives in one place, which I did in 84bb4aa.

Testing -- using the `tox.ini` in #64, I get:

```
vagrant@ubuntu:~/dev/git-repos/pycrypto$ tox -e py25,py26,py27,py32,py33,py34
...
  py25: commands succeeded
  py26: commands succeeded
  py27: commands succeeded
  py32: commands succeeded
  py33: commands succeeded
  py34: commands succeeded
  congratulations :)
```