virtualenv: remove depends_on six

[dulmandakh]

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virtualenv 20.16.0 dropped support for Python 2.x in pypa/virtualenv#2382, so this PR removes dependency on six, which is required to support Python 2.x. Please see https://github.com/pypa/virtualenv/releases/tag/20.16.0

[BrewTestBot]

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