python-pyqt: fix install

PyQt4 provides two version for some py files: one for python2.x and one for
python3.x. This two versions are installed in TARGET_DIR.
After installing files, python is used to generate all .pyc files, but fails
like :
class ProxyBase(metaclass=ProxyMetaclass):
                             ^
							 SyntaxError: invalid syntax

This is not a package build failure and this fix is only cosmetic.
This patch suppress directory for not targeted python version.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>