Drop 2.6 and 3.2 from Travis tests
Remove extra assert methods needed for Python 2.6
Update docs for Python version support
Drop nested() function for context managers
Remove _total_seconds helper function
Remove Python 2.6 workaround
remove workaround for 2.6 support
irunner use python -m IPython to launch subprocess with matching interpreter

Closes gh-4013
Merge pull request #4002 from takluyver/drop-26-32

Drop Python 2.6 and 3.2