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.
Well, I don't want to encourage it. :smile: 

I'll tone down the wording. I assumed that eventually 2to3 support in `astropy.tests.helper.TestRunner` would eventually be depreciated and removed when no longer needed, but I guess that's a list-worthy topic over multiple releases.