remove system python libs from path by default

[PierrickKoch]

commits 8365025 and 4fdcae6 allow to run morse with blender from http://download.blender.org/release without conflicting with local python libs, with the following environment variables set:

export MORSE_SILENT_PYTHON_CHECK=1
export MORSE_POP_PYTHONLIB="/usr/lib/python3.5"

We might want to remove system python libs by default, with something like:

sys.path = filter(lambda p: not p.split('/')[-1].startswith('python'), sys.path)

Though, this might be a problem if Blender was installed from source or with system package manager, in which case Python is not packaged with it. We might figure out a simple test for that...