Update Python versions

[hugovk]

Like #71, but one year on :)

Rather than removing EOL 3.2 and adding new 3.6, this removes EOL 3.3 (since 2017-09-29) and adds new 3.7.

Python 3.7 needs a little workaround on Travis CI to use Xenial with sudo, see
travis-ci/travis-ci#9815.

This also adds python_requires to setup.py to help pip.

I've left EOL Python 2.6 in place, is it still needed for RHEL 6?

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By the way, here's the pip installs for ecdsa from PyPI for August 2018:

python_version	percent	download_count
2.7	70.70%	687,544
3.6	17.94%	174,475
3.5	7.17%	69,774
3.4	2.18%	21,182
3.7	1.42%	13,849
2.6	0.56%	5,403
3.3	0.02%	195
3.2	0.00%	44
3.8	0.00%	16
None	0.00%	4
Total		972,486

Source: pypinfo --start-date 2018-08-01 --end-date 2018-08-31 -md --percent ecdsa pyversion

[coveralls]

Coverage remained the same at 86.095% when pulling ce9ebcd on hugovk:update-versions into 0d3d29e on warner:master.

[tomato42]

I just fixed CI on 3.3 so am not so eager to drop it 😃 (also, I'm using this package with tlsfuzzer and tlslite-ng, which still officially support 3.2 because of Debian wheezy)

In other words, unless there's a good reason to drop 3.3 (except its upstream support status), I am willing to do the extra work to keep CI and python-ecdsa working on 3.3. Given that 2.6 isn't going away, I don't think dropping 3.3 gives us access to any additional features (please correct me if I'm wrong, I haven't researched it).

adding 3.7 to the testing mix is much appreciated though

[hugovk]

Yeah, there's nothing much special to be gained from dropping 3.3, except not needing to worry about pinning to outdated dependencies, and freeing a CI slot.

I've dropped the 3.3 dropping :)

[tomato42]

@hugovk thanks!

[tomato42]

BTW: Support for RHEL-6 ends in 2024, so yes, py2.6 support will be needed for quite some time still.

[hugovk]

I see extended EOL of RHEL 5 is 2020, but I don't see many projects maintaining support for Python 2.4.

I wonder, is the adoption of RHEL 5 is much lower compared to RHEL 6, or something else?

Thanks!

[tomato42]

for RHEL 5 there already is a pre-packaged Python 2.6 in EPEL: https://ftp-stud.hs-esslingen.de/pub/Mirrors/archive.fedoraproject.org/epel/5/x86_64/python26-2.6.8-2.el5.x86_64.rpm