Expand tox tests to include recent python 3 releases

[godber]

I just noticed that your python 3 tests only go up to 3.4, 3.5 and 3.6 have since released and are in broad use.

https://github.com/planetarypy/pdsview/blob/master/tox.ini#L2

[percurnicus]

With the new setup, tox will be more difficult...but why does tox need to test more than one python3 version? Aren't they backwards compatible so I should only test 3.6?

[godber]

You make a good case and I would agree that it makes the most sense to test the most recent version if you're only testing one. Maybe in an ideal world you'd test them all. Again, your call at this point 😈

Edit: wording

[percurnicus]

So I've been going back and forth between what would be a better: only having py2 and py3 as environments which (I think) will pull from the most up to date python2 and python3 on the system or to include many environments and the setting skip_missing_interpreters=True so the tests pass when developers don't have all/multiple versions of 2.x and 3.x. I'm leaning toward the skip_missing_interpreters option because there are somethings that are not full backwards compatible between python 3 versions (Like formatted string literals). Not sure if you have an opinion on this