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.
This should speed it up a bit, as brew will download a pre-compiled "bottle", whereas pyenv compiles python from source.
The current 'python3' formula on homebrew is 3.6.0, so let's use that.
https://travis-ci.org/anthrotype/fonttools/jobs/123273641
When I do `pip install -r dev-requirements.txt` from within a virtual env,
jython sits for 3-4 minutes doing nothing, then the Travis shell exits with this cryptic error:
/usr/bin/env: python2.7: Argument list too long
