https://tox.readthedocs.io/en/latest/config.html#conf-skip_missing_interpreters
Runing `tox` with no options runs the tests agaist all the python
environments listed in the `tox.ini`'s `envlist` (currently 3.6, 3.7 and 3.8).
Before this change, if any of these versions was not available, tox would exit with an error. Now it will simply continue (with a warning).
This can be useful when on a developer box, one might only have a subset of all our supported interpreters installed but we don’t want to mark the build as failed because of it.
Note that on the CI I am passing the opposite command line switch to override this setting, because there I want to make sure none of the specified interpreters is skipped.
c1f6b4d0f1