changes_for_3

Changes made by John Lynch, May 2017 to adapt for Python 3.6

No changes to:
* error.py
* line.py


* __init__.py
  add import _builtins

* setup.py
aim to support both 2.7 and 3.5+ by switches in modules
  so remove python>=2.6 from install_requires

* shell.py, _spawn requires a correction to try except syntax 
    plus removal of kwargs from self.proc (compile error)
* also changed error to OSError as from OSError,

* builtins.py
  import __builtins__ with 3+ so change filename to _builtins.py
    and add > 2 test in imports (also defines basestr = str for 2+)
  ## dont know why line 47 uses warn( ????) 

* sequence.py
  import sys and redefine ixxx to xxx for python > 2
  rebuild defined_items for python > 2 as test uses iterator

* main.py
  a lot of changes including exec, list on map 
  problems with both 2 & 3 exec variants in 1 file lead to 2 new files
    & import to create _exec