Fix documentation build for Python 3

The documentation doesn't currently build under Python 3. The immediate error is due to use of `execfile` in the Sphinx configuration, but there may be others.

```
mirzakhani:docs mdickinson$ make html
sphinx-build -b html -d build/doctrees source build/html
Running Sphinx v1.8.3

Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/sphinx/config.py", line 368, in eval_config_file
execfile_(filename, namespace)
File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 150, in execfile_
exec_(code, _globals)
File "/Users/mdickinson/Enthought/ETS/codetools/docs/source/conf.py", line 44, in
execfile(os.path.join('..', '..', 'codetools', '__init__.py'), d)
NameError: name 'execfile' is not defined

make: *** [html] Error 2
```