build on Python 2.6 broken and strongly suggested minimum version

[James Browning]

Selsky broke the build[1] back in May 2020 when switching the
python_scripts variable in /wscript to a set when Python 2.6
does not support sets. Given the lack of complaints, I suspect
no one uses 2.6 anymore, which can be dropped.

I also think that we should drop all Python versions before
3.7 from first-tier support and only continue supporting them
if it is not inconvenient or there is sufficient proven
demand.

[1] 574d871ed 2020-05-01 "Use a set instead of a list for
python_scripts so we don't have duplicates"
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