sip fix HEAD

This fixes 2 problems when building sip with `--HEAD`:
* if build with both python versions, creating the symlink will fail on the 2nd run (already exist)
* build.py doesn't run under python3:
```
==> python3 build.py prepare
Traceback (most recent call last):
  File "build.py", line 618, in <module>
    action_names = [action.func_name[1:] for action in actions]
  File "build.py", line 618, in <listcomp>
    action_names = [action.func_name[1:] for action in actions]
AttributeError: 'function' object has no attribute 'func_name'
```