SIngle-sourcing the package version: Remove a py2-only option; simplify a py3 option (#605)

* SIngle-sourcing the package version: Remove a py2-only option

Thanks for this guide! I was using this and tried `execfile()` without realizing it didn't work in Python 3.

I thought about leaving the Python 2 version here with a disclaimer, though this list is already very long, and it seems more helpful to steer people toward a good solution than to exhaustively document an obsolete solution.

* Simplify py3 version

* Update source/guides/single-sourcing-package-version.rst

Co-Authored-By: paulmelnikow <github@paulmelnikow.com>

Author: paulmelnikow

source/guides/single-sourcing-package-version.rst

@@ -48,16 +48,6 @@ number of your project:
     your project (e.g. :file:`version.py`), then have :file:`setup.py` read and
     ``exec`` the value into a variable.
 
-    Using ``execfile``:
-
-    ::
-
-        execfile('...sample/version.py')
-        # now we have a `__version__` variable
-        # later on we use: __version__
-
-    Using ``exec``:
-
     ::
 
         version = {}