Update to Python 3.10 as default.

[icemac]

Add 3.11 as future and move 3.6 to legacy as its sunset is in December of this year.

[dataflake]

It just occurs to me that lumping together Python 2.7 and 3.6 as "legacy Python" means you can only have both together. So packages that are Python 3-only but still want to fully support Zope 4 will break.

IMHO Python 2 and Python 3 should not be in the same category. The with legacy python switch should be broken up into one that means "Python 2 support" and a separate one that means "older Python 3 support".

[icemac]

@dataflake If I remember correctly Zope 4 goes into security fix only mode by the end of this year. My expectation would be that it only receives security fixes from next year on, so we do not have to maintain Python 3.6 compatibility in newer feature releases of the packages for Zope 4. Python 3.6 has its sunset in December 2021, so I think it is okay to start dropping support for this version in newer releases of our packages together with other officially unsupported versions.

If I remember correctly there is currently just one package with with-legacy-python = false and I won't start dropping Python 2 support (+ support for Python 3.5 and 3.6) before 2022.