Some change to the Travis build agents was causing installation of the Python-based httpbin server (used for testing the http client) to fail and causes all of our Travis CI builds to fail with message:
virtualenv 20.0.1 has requirement six<2,>=1.12.0, but you'll have six 1.10.0 which is incompatible.
Some change to the Travis build agents was causing installation of the Python-based httpbin server (used for testing the http client) to fail and causes all of our Travis CI builds to fail with message:
This failure revealed that that our usage of tox to create a virtualenv is not recommended on TravisCI. The travis.yml was modified to avoid tox by running commands directly (where Travis has already configured a virtualenv).
As part of this change I enabled the use of python 3 as python 2 is deprecated as of Jan 1, 2020.
To enable python 3 usage I had to upgrade our CPPLINT version where I switched to the version maintained on https://github.com/cpplint/cpplint.
These changes currently enable developers to use either python 2 or python 3 (in particular the appveyor windows build still uses python 2).