Drop Python 2.7 support for master.

[p12tic]

Reasonings:

• Python 2 is going unmaintained as soon as 2020: https://pythonclock.org/

• Maintaining compatibility for both python2 and python3 is expensive resource wise

• Will allow to use python3 features like async/await, which adds a big performance boost for asynchronous apps like Buildbot

Timeline

Buildbot 2.0 is planned to be released on February 2019.
This version will drop support for 2.7 in the master.

Buildbot 1.7 and 1.8 will contain loud warning for python2 users at Buildbot start to invite them to upgrade.

Original issue:

There have been some talks about no longer supporting running master under python 2.x 
some time in the future.
 As 2.0 version will be released soon, maybe it would make sense to mark python 2.x support 
deprecated so that it can be removed in the next major release - 3.0?

[tardyp]

I would rather drop python 2.x for 2.0. This would allow us to switch to async/await and improve a lot readability debugability and efficiency.

What should be feasible imho is to have a installer script which would automatically convert async/await code to inlinecallbacks for compat with 3.5.

[p12tic]

Oh, even better. The async/await conversion is a great idea!

[tardyp]

Let's keep it open for discussion. I created an email to users@buildbot.net

[p12tic]

From email:

One solution would be to release a 1.7.0 LTS version, and only integrate bug fixes there.

I would vote for this approach. Given that buildbot has quite good test coverage, it wouldn't be hard to maintain it except for rewriting python 3.x-specific stuff for 2.7 when backporting.

[p12tic]

Thinking about it more, if we decided to have an LTS, wouldn't be dropping older python 3.x versions that don't support async/await a good idea from maintainability perspective too? Python 3.5.0 is already 3 years old and all LTS releases of major Linux distributions have at least that version.

Ubuntu 18.04 LTS: python 3.6.5
Debian stretch: python 3.5.3
Red Hat Enterprise Linux/CentOS 7: python 3.6.6 via EPEL

Any users who can't upgrade could just use 1.7.x.

[seankelly]

RHEL 8 had a beta release with:

Python 3.6 is the default Python implementation in RHEL 8; limited support for Python 2.7 is provided. No version of Python is installed by default.

[rodrigc]

We had some discussion for ideas on dropping Python 2.7 support here:

https://supybot.buildbot.net/meetings/buildbot/2018/buildbot.2018-09-04-16.00.log.html#l-100

[rodrigc]

I suggested a timeline for dropping Python 2.7 support: https://lists.buildbot.net/pipermail/users/2018-November/001756.html

I also recommended updating the https://buildbot.net web site to put a warning deprecating Python 2.7 support

[tardyp]

Looking at the telemetry, we still have 70% of the significant installs (> 1 builder) which is python 2.7, seen over the last 90 days.

I am adding a warning at Buildbot start
#4469

Some suggested also on the documentation, which is the most looked at content, probably more than
the marketing site.

[rodrigc]

Not surprising, if pip install buildbot is what people do, that will install buildbot with Python 2.7.