Removing dependence on six

[john-science]

What is the change?

Removing ARMI's direct dependence on six.

However, you will notice I didn't remove six from pyproject.toml, because it is not in there. One of ARMI's secondary dependencies is six, apparently. (I haven't bothered to figure out which one.)

Why is the change being made?

ARMI doesn't support Python 2 any more, so we don't need this dependence.

close #2036

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes have been updated if necessary. (This doesn't require a release note.)
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[john-science]

Okay, since this PR / ticket exists because we had a first-order dependency masquerading as a second-order dependency, I have gone through every import in ARMI to make sure they are included in our pyproject.toml file and otherwise make sense.

Here are the results:

• armi/utils/parsing.py: from dateutil import parser
  • First-order dependency not in pyproject.toml!
• armi/bookkeeping/tests/test_historyTracker.py: import nbformat
  • (also used in armi/tests/test_notebooks.py)
  • First-order dependency not in pyproject.toml!

[drewj-tp]

🧹