open-babel: Add HEAD and tests

Add HEAD option and exclude various patches and install hacks that have been fixed upstream. Also includes improved python detection and some simple tests.
I believe the layout of the Python installation can vary depending on whether it is from the system, homebrew, anaconda, etc. I originally got that strategy from the opencv and vtk formulas ages ago, but checking now they have simplified to just use `#{pypref}/lib/libpython#{pyvers}.dylib` so I assume that is fine in all cases. I'll update.