Use python 2.7+ instead of 3.6 in mail/claws-mail

[joneum]

Summary

After r489852, i talk with koobs on irc.
He suggested using python better: 2.7+. So the user can decide he wants Python 2.7 or higher.
I think that's a very good idea.
I open this review to discuss this with mmokhi as well.
This review is for the change from 3.6 to 2.7+. The poudrire tests look good:
http://joneumbox.org/build.html?mastername=112amd64-ports&build=2019-01-10_10h31m28s
http://joneumbox.org/build.html?mastername=112i386-ports&build=2019-01-10_10h31m28s

http://joneumbox.org/build.html?mastername=120amd64-ports&build=2019-01-10_10h36m18s
http://joneumbox.org/build.html?mastername=120i386-ports&build=2019-01-10_10h36m25s

http://joneumbox.org/build.html?mastername=13amd64-ports&build=2019-01-10_10h41m07s
http://joneumbox.org/build.html?mastername=13i386-ports&build=2019-01-10_10h41m33s

koobs also suggested to use "dm" instead of "m" in the patch. There is an error here:
http://joneumbox.org/build.html?mastername=120amd64-ports&build=2019-01-10_10h03m54s

I am not sure: when changing the Python version, does PORTREVISION need to be bump?

[mmokhi]

Hi,
About python27 compatibility I think it's good idea definitely
(It's good that you tested it and it's fine too 😊 I think the changes on the previous commit were mostly compatible).

For the m/dm stuff, I'm actually not very sure what @koobs meant, but
I propose this change (which tested and seems working for me),
so if you both can verify I appreciate 😄
I thinks it covers now all python versions (and all m/dm/blanks).

[mmokhi]

Oh and about the PORT_REVISION I support changing it, because we're making change on configure.ac of claws-mail
So sub-ports should get notified. It's safer decision imho :)

[koobs]

It's not that 2.7+ is "better", its that :3.6 is "incorrect" in that:

• It forces one specific version unnecessarily
• It doesn't accurately/precisely describe the versions the software does supports (basically: any version)

The pattern/guidelines is:

• The Python ports version specifier should be the set of Python versions that the package supports, not what Python version should be used (declarative vs imperative). Unfortunately the distinction is made more difficult to grok because of the name of the USES directive, which has an imperative meaning.

What this means in practice is:

• Don't specify one version unless the package *only* supports that (and only that) one specific version. There are practically no python packages in the world that are like this. For python "ports".

Also, please make sure this gets to quarterly too (MFH)