SCHWED-4715: Get rid of Python 2.7 as a string as far as sensible

core/init/__init__.py.in

doc/contributing.rst

extras/scoring_weight_training/run_all_optimizations.py

extras/scoring_weight_training/run_array_job.sh

sidechain/doc/rotamer.rst

sidechain/doc/rotamer_constructor.rst

sidechain/doc/rotamer_lib.rst