py3: conditionalize the raise statement
raise E,V,T is not acceptable in Python 3, thats is conditionalized.
Moreover this will result in syntax error so we have to use exec() to
execute this. Related PEP- https://www.python.org/dev/peps/pep-3109/#id14
My implementation is motivated from the six implementation except they are
defining a new function exec_() to prevent adding an extra frame AFAIK :)
https://bitbucket.org/gutworth/six/src/ca4580a5a648/six.py#six.py-680