Remove old Pythons from CI

Currently the CI jobs targeting Python 2.7 and Python 3.3 are failing because the archives for installing these Python versions no longer exist.

The log looks like this:
```
Downloading archive: https://storage.googleapis.com/travis-ci-language-archives/python/binaries/ubuntu/16.04/x86_64/python-2.6.tar.bz2
travis_time:start:1366dd53
$ curl -sSf --retry 5 -o python-2.6.tar.bz2 ${archive_url}
curl: (22) The requested URL returned error: 404 Not Found
travis_time:end:1366dd53:start=1604439964106137863,finish=1604439964208478148,duration=102340285,event=configure
Unable to download 2.6 archive. The archive may not exist. Please consider a different version.
```