Drop support for EOL Python 2.6

[hugovk]

Summary: yamllint used to support Python 2.6 because it's a dependency of Ansible, which supported Python 2.6. Ansible has now dropped 2.6, so yamllint may also drop it.

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Python 2.6 is EOL since 2013-10-29 and no longer receiving security updates (or any updates) from the core Python team.

Source: https://en.wikipedia.org/wiki/CPython#Version_history

Ansible 2.7.0 was released in October 2018 which drops Python 2.6.

• https://deleforterie.com/wordpress/index.php/2018/10/21/ansible-python-2-6-support-ends-with-ansible-2-7/
• https://github.com/ansible/ansible/releases/tag/v2.7.0
• Exorcise Python-2.6 ansible/ansible#42971

• Support for running an Ansible controller with Python-2.6 has been dropped. You can still manage machines which use Python-2.6 but you will have to manage them from a machine which has Python-2.7 or Python-3.5 or greater installed. See the porting guide if you need more information.

https://github.com/ansible/ansible/blob/stable-2.7/changelogs/CHANGELOG-v2.7.rst#v2-7-0

The decision to drop Python-2.6 support on the controller was made because many dependent libraries are becoming unavailable there. In particular, python-cryptography is no longer available for Python-2.6 and the last release of pycrypto (the alternative to python-cryptography) has known security bugs which will never be fixed.

https://docs.ansible.com/ansible/latest/porting_guides/porting_guide_2.7.html#python-compatibility

Yamllint's support of Python 2.6 is also preventing its dependencies from dropping it too, for example pathspec: cpburnz/python-pathspec#16.

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This PR drops support for 2.6.

It adds python_requires to help pip install the right version for people still running old Python versions, plus Trove version classifiers for PyPI.

It also upgrades unit tests to use more useful asserts, and upgrades to modern Python syntax using pyupgrade.

[coveralls]

Coverage remained the same at 98.185% when pulling d0ac862 on hugovk:rm-2.6 into 8bdddf6 on adrienverge:master.

[adrienverge]

Ansible has now dropped 2.6, so yamllint may also drop it.

yamllint is not only used by Ansible. What makes you say that Python 2.6 support can be dropped like that, without impacting any other project?

[hugovk]

I understand if you wish to retain 2.6, and the decision is of course ultimately yours.

However, at some point it's good to draw a line and stop supporting EOL versions, to reduce maintenance burden, to allow newer features of Python to be used, to reduce resource usage, and also free other dependencies from supporting old versions.

Two CPython core developers talk about the network effect here and here.

I think now is a good time to drop 2.6. People still using 2.6 are advised to upgrade to 3.x (or 2.7). Those still using 2.6 are already using an insecure Python runtime and already have to pin many dependencies which have dropped 2.6.

I agree completely that Ansible isn't the only thing using yamllint. And there's no way to know what proprietary/in-house/closed-source code might still be using yamllint with Python 2.6 (or 3.3!).

Ansible was cited as the reason to maintain 2.6 support in, for example, #55 (comment) and cpburnz/python-pathspec#16.

The latter also says yamllint is used by OpenStack, however that has also dropped 2.6: https://wiki.openstack.org/wiki/Python3#Python_2:_Python_2.6_support_dropped.2C_Python_2.7_only.

Looking at the first 10 projects under "Used by" at https://libraries.io/pypi/yamllint, I don't see any supporting Python 2.6:

Project	GitHub stars	Support 2.6?
ansible/ansible	33764	No
ansible/molecule	1628	No
Neo23x0/sigma	768	No
ansible/galaxy	424	No
coala/coala-bears	221	No
markstory/lint-review	177	No
IBM/Kubernetes-container-service-GitLab-sample	148	No
mangaki/mangaki	79	No
bosondata/badwolf	64	No
redhat-openstack/infrared	42	No

Finally, with Python 2's EOL approaching, many projects have pledged to drop support for Python 2 on or before 2020-01-01

• https://python3statement.org
• https://pythonclock.org

[adrienverge]

Thanks a lot for this comprehensive review of dependent Python projects. You convinced me.

I'll do a slight reword of your last commit then merge.

[hugovk]

You're welcome, and thanks for your work on yamllint!