Remove Python 2 compatibility shims

[galenlynch]

Overview:

six and future are Python 2 compatibility shims. Python 3 was released 18 years ago. AllenSDK now requires Python 3.10+. Time to stop using Python 2.

Addresses:

Drop 18+ year old python code.

Type of Fix:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to not work as expected)
• Documentation Change

Solution:

Remove all Python 2 compatibility shims (six and future packages) and replace with stdlib equivalents.

Changes:

Remove all import six / from six import ... / from past.builtins import ...
across the codebase and replace with stdlib equivalents:

Pattern	Replacement
six.iteritems(d)	d.items()
six.itervalues(d)	d.values()
six.string_types	str
six.text_type	str
six.moves.xrange	range
six.moves.reduce	functools.reduce
six.moves.builtins	builtins
six.moves.cPickle	pickle
six.raise_from(X, e)	raise X from e
six.ensure_str(s)	s (already str in Python 3)
past.builtins.xrange	range

Drop six and future from pyproject.toml dependencies.

Validation:

• pytest: 2484 passed, 207 skipped, 1 xfailed, 0 failures
• Notebooks: 30/30 passed
• Tested with Python 3.13 in a clean venv

Checklist

• My code follows
  Allen Institute Contribution Guidelines
• My code is unit tested and does not decrease test coverage
• I have performed a self review of my own code
• My code is well-documented, and the docstrings conform to
  Numpy Standards
• I have updated the documentation of the repository where
  appropriate
• The header on my commit includes the issue number
• My Pull Request has the latest AllenSDK release candidate branch
  rc/x.y.z as its merge target
• My code passes all AllenSDK tests