TESTED_VERSIONS is ignored in tox?

I set custom `TESTED_VERSIONS` in astropy/astrowidgets#103 but it does not seem to work, as the test log still shows:

```
Running tests with Astropy version 4.0.
Running tests in astrowidgets docs.
```

It works in my other projects. The only difference I can think of is that `astrowidgets` is using `tox`, unlike the others that I am using for comparison.

Read the tox configuration used by astrowidgets and compare how it passes TESTED_VERSIONS with the configuration in a project where the setting works. Reproduce the test run and verify that the log uses the requested versions instead of Astropy 4.0.

Written by the indexing model from the issue text.