Fix documentation build failing on Python 3

[kitchoi]

When I tried to build the documentation with the following commands:

# In an environment with enable source installed
pip install sphinx
cd docs
make html

I get the following error:

Traceback (most recent call last):
  File "/Users/kchoi/.edm/envs/enable-test-3.6-null-pillow/lib/python3.6/site-packages/sphinx/config.py", line 319, in eval_config_file
    execfile_(filename, namespace)
  File "/Users/kchoi/.edm/envs/enable-test-3.6-null-pillow/lib/python3.6/site-packages/sphinx/util/pycompat.py", line 89, in execfile_
    exec(code, _globals)
  File "/Users/kchoi/Work/ETS/enable/docs/source/conf.py", line 42, in <module>
    execfile(os.path.join('..', '..', 'enable', '_version.py'), d)
NameError: name 'execfile' is not defined

execfile is a Python 2 built-in. This PR fixes the config file so that the config file works on Python 3. We probably want to add a docs command in ci/edmtool.py like in the other ETS projects to streamline this step, and possibly run this command as part of CI, but that can come in separate PRs/issues.

[codecov-io]

Codecov Report

Merging #478 (03bcd69) into master (2322ff2) will increase coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #478      +/-   ##
==========================================
+ Coverage   30.26%   30.45%   +0.19%     
==========================================
  Files         206      206              
  Lines       18143    17852     -291     
  Branches     2461     2461              
==========================================
- Hits         5491     5437      -54     
+ Misses      12309    12072     -237     
  Partials      343      343              

Impacted Files	Coverage Δ
enable/tools/move_tool.py	82.75% <0.00%> (-1.12%)	⬇️
enable/coordinate_box.py	31.15% <0.00%> (-1.04%)	⬇️
enable/component.py	54.64% <0.00%> (-1.03%)	⬇️
enable/abstract_window.py	53.43% <0.00%> (-0.84%)	⬇️
enable/interactor.py	84.00% <0.00%> (-0.62%)	⬇️
enable/component_editor.py	82.97% <0.00%> (-0.36%)	⬇️
enable/tools/drag_tool.py	77.17% <0.00%> (-0.25%)	⬇️
kiva/gl.py	0.00% <0.00%> (ø)
kiva/cairo.py	0.00% <0.00%> (ø)
enable/viewable.py	0.00% <0.00%> (ø)
... and 48 more

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[jwiggins]

LGTM