Revert "Add six to list of mocked modules"

This reverts commit 6ebf0fd3aa3c2b17a2ae324196f59c9e0e4614ab.

We need six installed in order for the docs to build. Six is too magical
to be mocked, at last, for now.
Merge pull request #18142 from s0undt3ch/hotfix/build-docs

Revert "Add six to list of mocked modules"