Drop support for EOL Python 2

[hugovk]

The Python 2.7 build is failing:

• https://travis-ci.org/thombashi/pytablewriter/jobs/646389612

It looks like it's because tox is running python setup.py test which is installing dependencies without using pip, and so python_requires is ignored, and so tries to install newest versions which don't support Python 2 any more.

I think python setup.py test is no longer recommended, and it would be better to do something like pip install . (or pip install -e .) and pytest etc.

Whilst that would still be a good idea, another idea is also to drop Python 2 support here.

Here's the pip installs for pytablewriter from PyPI for January 2020:

category	percent	downloads
3.7	75.12%	25,827
3.6	16.61%	5,711
null	3.44%	1,184
3.8	2.43%	835
3.5	1.56%	537
2.7	0.81%	280
3.4	0.01%	5
3.9	0.01%	4
Total		34,383

Source: pip install -U pypistats && pypistats python_minor pytablewriter --last-month

[thombashi]

Thank you for your pull request and information.

I had committed some workaround to fix CI for Python 2. However, I might need to reconsider test scripts.

I will review the code, probably this weekend.

[thombashi]

LGTM. Thanks.

[hugovk]

You're welcome, thanks for pytablewriter!