Post init hook is not run if validators are disabled

Here's the `__init__` code generated by attrs:
```python
def __init__(self, name, loc):
    self.name = name
    self.loc = loc
    if _config._run_validators is False:
        return
    __attr_validator_name(self, __attr_name, self.name)
    __attr_validator_loc(self, __attr_loc, self.loc)
    self.__attrs_post_init__()
```