[Snowball-discuss] Dropping Support Python 2

[Olly Betts]

I'm proposing to stop actively supporting Python 2, which the Python
developers stopped supporting it at the start of 2020.  The particular
motivation for this is that the CI tests no longer work because the CI
system appears to no longer have a Python 2 build available, but it
seems an appropriate change at this point in time anyway.

This doesn't involve any code changes since the same generated Python
code works for both Python 2 and Python 3 - the change is really just
to the documentation (to say that only Python 3 is actively supported)
and to the CI config.

So we'll actually still be generating code that probably works for
Python 2, it's just that we'll now deem this an unsupported combination
and if it breaks we won't put effort into fixing it.  However if someone
submitted a clean patch I'd very likely still merge it.

Cheers,
    Olly