Test multi python versions

[vladimir-v-diaz]

securesystemslib officially supports Python 2.7, 3.3, 3.4, 3.5, and 3.6.

It appears that we cannot currently run CI tests under Python 3.3 (interpreter not found) and Python 3.5 due to issues with Travis. For now, this pull request ensures testing under Python versions 2.7, 3.4, and 3.6.

We should enable testing under Python 3.5 and possibly 3.3 (it is nearing its End of Life) once Travis resolves the current problems with Python interpreters.

[coveralls]

Coverage remained the same at 100.0% when pulling fd609d4 on vladimir-v-diaz:test_multi_python_versions into b8c047d on secure-systems-lab:master.

[coveralls]

Coverage remained the same at 100.0% when pulling fd609d4 on vladimir-v-diaz:test_multi_python_versions into b8c047d on secure-systems-lab:master.

[vladimir-v-diaz]

I updated the pull request summary to note the inability to test under Python 3.3 and 3.5 due to issues with Travis at the moment. For now, we run Travis-ci tests for Python 2.7, 3.4, and 3.6.

[baloian]

Do you have an open issues for Python 3.3 and 3.5 support once Travis solves it's issue ?
Open an issue if you think it makes sense.

[vladimir-v-diaz]

Great point. I will open an issue so that we can keep track of it.