ansible: add python3 to AIX hosts

[richardlau]

I've run this on all of our test/release AIX 7.1 and 7.2 hosts. It installs Python 3 (3.7.9 at this time) into /opt/freeware/bin, e.g. /opt/freeware/bin/python3. Crucially it leaves the existing Python 2 installation alone (so python still runs Python 2).

On the AIX 7.2 machines there is already a Python 3.6.12 installation in /opt/bin/python3. We end up with three versions of Python installed (2.7.18, 3.6.12 and 3.7.9) but they're in different directories and the python3 binary we pick up depends on the path. If we've run through select-compiler.sh and prepend /opt/freeware/bin to the path python3 will be 3.7.9.

build/jenkins/scripts/select-compiler.sh

Line 103 in f95880c

export PATH="/opt/ccache-3.7.4/libexec:/opt/freeware/bin:$PATH"

I haven't found a CI job that will actually use Python 3 yet as they either directly call python configure ... (i.e. invoke python which is Python 2) or they run make run-ci which defaults the PYTHON make variable to python (again Python 2). We'll probably need to directly run configure ... (as there is Python autodetection/selection logic in the configure script) and/or set the default PYTHON in the makefile to python3 in master/16.x onwards.

cc @nodejs/platform-aix

[mhdawson]

LGTM

[richardlau]

Refs: #2507