Remove a bunch of Python2-related warts (#2315)

* remove all of the "from __future__" imports

* remove the first batch of rdkit.six imports/uses

* next step of rdkit.six removal

* removing xrange, range, and some maps

* next round of removals

* next round of cleanups

* fix inchi test

* last bits of "from rdkit.six" are gone

* and the last of the six stuff is gone

* strange importlib problem

Scripts/PythonFormat.py

@@ -11,7 +11,7 @@ all non-conformant files are listed and the required yapf command(s) printed.
 If changes are found, the script will exit with error code 1, otherwise 0.
 
 '''
-from __future__ import print_function
+
 import os
 from yapf.yapflib.yapf_api import FormatCode
 import sys