support PEP-3149 when installing python-extensions

Python extensions, like `mpi.so` should not be staged as `mpi.so` but instead should be staged as e.g. `mpi.cpython-32mu.so` where the suffix or tag can be figured out with

```
>>> sysconfig.get_config_var('EXT_SUFFIX')
'.cpython-32mu.so'
>>> sysconfig.get_config_var('SOABI')
'cpython-32mu'
```

Such that PEP-3149 becomes supported, and one can trivially reuse the same stagedir for multiple python builds.