Ported sdrmsd and sdtether to Python 3 and Open Babel 3

Cleaned up inconsistent use of tabs and spaces and replaced old style
string formatting (%) with new style (.format()). Used autopep8 for
further PEP 8-compliant style fixes.

Open Babel 3 puts pybabel module inside openbabel, so import has been
changed to try that first and fall back to 2.x style import if the first
import fails.