Remove the use of six package.

[mcepl]

The package aspires to compatibility with Python >= 3.6, so compatibility with Python 2 is not needed any more.

The commit strive to be completely functional neutral, no other changes were made in the code.

Fixes: https://github.com/oracle/oci-cli/issues/984

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[mcepl]

I am an SUSE employee, so I need to ask our legal team, whether we have signed OCAA.

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