python3: fix the value of PYTHON3_PATH

The PYTHON3_PATH was incorrectly referencing the site-packages of
Python 2 packages, due to the usage of PYTHON_VERSION_MAJOR, instead
of PYTHON3_VERSION_MAJOR. This commit fixes that by using the correct
variable, which in our testing fixed the build of python-pyasn against
python3.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>