dev-python/numba-0.39.0.ebuild support only:
PYTHON_COMPAT=( python{2_7,3_5,3_6} )
Based on setup.py it support python 3.7 as well.
After modifing the ebuild, by adding
PYTHON_COMPAT=( python{2_7,3_5,3_6,3_7} )
It worked fine