Add support for brewed Python in Ice 3.3 formula

With this PR locally merged, the three following scenarios should be testable:
- System-wide Python

 ```
$ brew install zeroc-ice33
$ PYTHONPATH=$(brew --prefix zeroc-ice33)/python /usr/bin/python -c 'import Ice'
```

- Brewed non-framework Python

 ```
$ brew install python
$ brew install zeroc-ice33
$ PYTHONPATH=$(brew --prefix zeroc-ice33)/python /usr/local/bin/python -c 'import Ice'
```
 The following command
 ```
$ PYTHONPATH=$(brew --prefix zeroc-ice33)/python /usr/bin/python -c 'import Ice'
```
 should fail since IcePy is build against the brewed Python

- Brewed framework Python

 ```
$ brew install python --framework
$ brew install zeroc-ice33
$ PYTHONPATH=$(brew --prefix zeroc-ice33)/python /usr/local/bin/python -c 'importIce'
```

 Also

 ```
$ PYTHONPATH=$(brew --prefix zeroc-ice33)/python /usr/local/bin/python -c 'import Ice'
```
 should fail since IcePy is build against the brewed framework Python