build: update prerequisites on progress towards Python 3

[cclauss]

As recommended at #25759 (comment)

• Python 2.6 end of life statement in 2013: https://www.python.org/dev/peps/pep-0361/#release-lifespan
• Python 2.7 end of life statement in 2019: https://www.python.org/dev/peps/pep-0373/#update
• Python 3.4 reaches it end of life in < 50 days so it should not be a target: https://devguide.python.org/#branchstatus

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[Trott]

@nodejs/build @nodejs/documentation

[Trott]

Lite CI: https://ci.nodejs.org/job/node-test-pull-request-lite-pipeline/2424/

[Trott]

@nodejs/python

[Trott]

Lite CI: https://ci.nodejs.org/job/node-test-pull-request-lite-pipeline/2425/

[refack]

LGTM as a semver-major move for node12

[Trott]

@nodejs/tsc

[ofrobots]

This is a FYI doc-only change. Why is this semver-major?

[refack]

Why is this semver-major?

I think that a formal change in supported toolchains (i.e. no longer supporting Python2.6) should be semver-major.
Effectively this frees us to break compatibility with 2.6, and to stop testing with 2.6.

[ofrobots]

Ack. I missed the 2.6 drop the first time around. Sorry about that.

LGTM.

[refack]

I think semver-major/"breaking change" should be applied only to things that might break stuff for the end user.

FWIW, some end users do compile their own binary, and some users are downstream consumers in some way (embedders / tinkerers), so in that sense a change in the toolchain could break their usage.

[Trott]

Even though this is going in as semver-major, I imagine we don't need a CITGM run for this, so I'll add the author ready label.

[addaleax]

Landed in bb564a3