python3: use proper PYTHON3_VERSION_MAJOR instead of harcoding 3.3

Some parts of python3.mk were hardcoding the 3.3 version as the major
version, which does not work for Python 3.4 and other future
versions. Instead, use the existing PYTHON3_VERSION_MAJOR.

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