Fix call to checklib.py from comparelibs.py (--nocolor arg and python3)

Created by: justyn

Fixing 2 bugs in comparelibs.py when it called checklib.py:

• Using --nocolor along with extra arguments (eg --footprints) would lead to concatenation, and would therefore fail with eg: checklib.py: error: unrecognized arguments: --nocolor--footprints

• checklib.py was still called with python by comparelibs.py instead of python3, leading to python: not found if python is not available on the system, as is the case for Ubuntu 20.04. Availability of python3 is already enforced in both files by their shebangs.