Python updates, news, and policy

[Joshua Root]

Here are a few Python-related FYIs and one proposal to solidify our policy.

Python 3.6 reached end-of-life as of 2021-12-23. Please feel free to 
start removing use of it from ports, and accelerate removal of use of 
3.5 and older. But *please* remember to only remove py36 subports (and 
indeed any ports) *after* they have no remaining dependents.

Python upstream has switched to an annual release cycle[1]. This means 
that new major versions will always be released in October going 
forward. I propose that we sync up with this schedule and change the 
default Python version in MacPorts, i.e. the default value of 
python.default_version in the python portgroup and the recommended 
default variant for ports that offer pythonXY variants, to the latest 
upstream release every year on the 1st of January, unless there are 
known issues that would make this significantly problematic. This timing 
means that the new major version will have had its first bugfix release 
for a few weeks, giving time for evaluation; plus it has the advantage 
of being easy to remember. :)

Setuptools has deprecated 'setup.py install', which is currently the 
default installation method used by the python portgroup. It still works 
fine for now, but at some unspecified point in the future it will be 
going away, so all ports using the portgroup need to start being 
migrated to use the new PEP 517 style builds. This is done by setting 
'python.pep517 yes' in the Portfile. That option is only supported with 
Python 3.6 and newer, so ports that still support older pythons will 
need to set it conditionally based on the python version.

Setting python.pep517 will add dependencies on an appropriate build 
front-end automatically, but each port will need to specify its 
dependencies for the back-end used by the project being built. Most will 
use setuptools, but others may use different back-ends such as flit or 
poetry.

Unfortunately, setuptools alone is not sufficient to do a pep517 build, 
and any ports that use setuptools as their pep517 back-end will need to 
additionally depend on pyXY-wheel. You also can't just build with 
distutils in a PEP 517 world (and distutils will also be disappearing 
from the stdlib in a future Python release.) So anything currently 
relying on distutils (i.e. no dependencies but pythonXY) will need to 
add dependencies on setuptools and wheel.

- Josh

[1] https://www.python.org/dev/peps/pep-0602/