Update Python versions

[hugovk]

Python 2.6, 3.2 and 3.3 are EOL and no longer receiving security updates.

They're also little used if at all.

Here's the pip installs for ConfigArgParse from PyPI for last month:

python_version	percent	download_count
2.7	88.83%	150,131
3.4	5.78%	9,762
3.6	3.08%	5,203
3.5	2.00%	3,379
2.6	0.25%	417
3.7	0.04%	63
None	0.03%	44
3.3	0.01%	18

Source: pypinfo --start-date -46 --end-date -19 --percent --pip --markdown ConfigArgParse pyversion

This removes those, and updates the code to use more modern Python features. It also adds Python 3.6.

[latk]

Edit: Python 2.6 support is no longer an issue for gcovr. My points about breaking downstream modules still hold, but I no longer have a horse in this race.

---

I am currently evaluating the use of ConfigArgParse for the gcovr tool. If this PR is merged and ConfigArgParse drops support for Python 2.6, I will have to find a different way to deal with config files.

I understand the desire to move to modern Python. However, some operating systems (CentOS 6) are still stuck on Python 2.6. CentOS 6 will be EOL'ed after Nov 2020. I have no desire to break hundreds of downstream users unless continuing to support 2.6 would require excessive effort.

This PR doesn't seem to provide any substantial value that would be impossible with continued support for Python 2.6. I would therefore suggest that the PR is not merged in its current form – although some of the small fixes and enhancements are nice and can be rebased/cherry-picked independently.

[hugovk]

I wouldn't say CentOS 6 is stuck on Python 2.6, I understand upgrading Python is officially supported by CentOS via the Software Collections Repository.

Enter Software Collections, also known as SCLs. As an example, SCLs allow you to run the default python that comes with CentOS (so yum and other system tools in CentOS work), while also allowing a newer version of python to be installed alongside the default python for use creating and running software with newer requirements.

https://wiki.centos.org/AdditionalResources/Repositories/SCL

And if upgrading from an old Python is unsuitable, pinning to an old release of this library is easily doable, as is already needed with other libraries for a 5-years-insecure 2.6.

Some Python libraries are already planning how to drop Python 2.7, which will be EOL on 2020-01-01.

See also:

• Unsupported since 2013-10-29
• https://snarky.ca/stop-using-python-2-6/
• http://www.curiousefficiency.org/posts/2015/04/stop-supporting-python26.html
• http://www.python3statement.org
• Current pip 9 deprecates Python 2.6 support, pip 10 won't support it (Drop support for Python 2.6? pypa/pip#3955)
• Wheel no longer supports Python 2.6 pypa/wheel@c9e5486
• Not much PyPI traffic (June 2016) Deprecate/Drop Support for Python 3.3? pypa/pip#3796

[hugovk]

Updated to add python_requires to help pip, and upgrade Python syntax using https://github.com/asottile/pyupgrade.

[bw2]

I like the changes in this PR, but it seems to me they're not necessary for any actual functionality. On the other hand, if there are 1+ users that still need python 2.6 for whatever reason, then merging it has negative value.

[bw2]

How many people still want python2.6 support?
Please reply or thumbs-up this comment if you do.

(@latk )
If no one replies during the next ~1 month, I'll merge this.

[latk]

@bw2 In the meanwhile I've dropped Python 2.6 support from gcovr, and have also decided to not use ConfigArgParse. As far as I am concerned, happy merging to you :)

[hugovk]

Rebased on master, removed the now-redundant sudo keyword from .travis.yml, and updated Python syntax with pyupgrade.

[ronhanson]

I'm ok to merge that. @bw2 ?

[hugovk]

Here's the pip installs for ConfigArgParse from PyPI for May 2019:

category	percent	downloads
2.7	79.05%	778,985
3.4	11.50%	113,298
3.6	4.98%	49,039
3.7	2.65%	26,161
3.5	1.58%	15,563
null	0.16%	1,590
3.8	0.05%	498
2.6	0.02%	225
3.3	0.00%	20
3.2	0.00%	2
Total		985,381

Source: pypistats python_minor ConfigArgParse --last-month # pip install pypistats