Add new formulas : pyqt-py3 and sip-py3

These formulas are copies of respectively 'pyqt' and 'sip', because
they are using python2. If you want to use PyQt with python 3, you have
to install 'pyqt-py3' instead of 'pyqt'.

Note :
- pyqt-py3 and pyqt are incompatible
- sip-py3 and sip are incompatible