Compatibility fix for Python3
remove six usage (unnecessary starting python 2.6)
download should work also for python3
Qt frontend: Handle 'aborted' prompt replies.

On some rare occasions, the execution request for prompt
is aborted by the kernel (due to an error in another execution
request), which is not handled gracefully by the ipython frontend
raising `KeyError: 'execution_count'`
This commit simply handles 'aborted' prompt requests by retrying it.
make davinci parallel example python3 compatible
simpler way to have python2 compatibility
simpler python2 compatibility for davinci code
proper python exception handling
Merge pull request #4794 from stonebig/python3_example_fix

Compatibility fix for Python3 (Issue #4783 )
Merge pull request #4800 from pankajp/handle-aborted-prompt-reply

Qt frontend: Handle 'aborted' prompt replies.
a minor typo in the code mirror stuff
document tab and shift-tab in codemirror