Improved plot digits classification example.

Made two for loop statements more readable by introducing local variables.
<MRG> Doc generation works in Python 2 and 3

Documentation can be generated using Python 2 or Python 3 and sphinx
[MRG] Addressed comments
[MRG] Addressed comment, fixed open-ended except

Except wile opening a URL now only catches AttributeError, eliminating an issue when
running Python 2 without internet connection.
More Py3 fixes for sphinx build
Merge branch 'python-3-sphinx-fixes'
Merge pull request #3063 from chalmerlowe/improved-digits-example

[MRG+1] Improved plot digits classification example.