purge six (even the vendored one) from non-vendored files

[MeggyCal]

As this library is Python 3 only, it shouldn't need six to run. It is a preparation for Python 3.13, however, some dependencies still need six.

Some files I edited by hand, but most work was done by sed scripts, namely:
sed -i '/import six/d'
sed -i 's/six.iter\([a-z]\+\)(\([_\.a-z]\+\))/\2.\1()/g'
sed -i 's/six.string_types/str/g' and friends (the same with binary_type, integer_types, etc)

If there is any mistake, typo, anything, let me know :) .

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[MeggyCal]

Well, I am reluctant to create an account... I will think about that.

[MeggyCal]

Anyway, I give anyone my permission to use those sed scripts (or even copy my commit) and basically bypass my PR. I just want this library to work even after six dies.