Remove/reword existing references to 2to3

Just trying to make documentation clear that 2to3 is no longer used in `astropy`. This is therefore a continuation of #2033.

I also removed the `test_run_after_2to3` test in this PR, and enabled the path fixing for coverage tests for python3.
I think this wording is too strong - I don't think we've decided yet to force affiliated packages to phase out 2to3.