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 :) .

[oracle-contributor-agreement]

Thank you for your pull request and welcome to our community! To contribute, please sign the Oracle Contributor Agreement (OCA).
The following contributors of this PR have not signed the OCA:

• PR author: MeggyCal
• [email protected] (@MeggyCal)

To sign the OCA, please create an Oracle account and sign the OCA in Oracle's Contributor Agreement Application.

When signing the OCA, please provide your GitHub username. After signing the OCA and getting an OCA approval from Oracle, this PR will be automatically updated.

If you are an Oracle employee, please make sure that you are a member of the main Oracle GitHub organization, and your membership in this organization is public.

[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.