Remove Python 2 artifacts from codebase

[Copilot]

Python 2 reached end-of-life on January 1, 2020. This PR removes all Python 2 compatibility shims and the future package dependency.

__future__ imports removed

All from __future__ import absolute_import, division, print_function, unicode_literals lines dropped from ~19 files — these are no-ops on Python 3.

Runtime compatibility branches eliminated

• basestring try/except → str
• sys.version[0] == '2' guards for functools, zip_longest, and izip_longest → direct Python 3 imports
• sys.version_info >= (3, 5) guard in helper.py → always use concurrent.futures.ThreadPoolExecutor
• try/except ImportError fallbacks from collections → direct collections.abc imports (Iterable, Mapping, Sequence)
• Python 2 __div__/__rdiv__/__idiv__ block in _compat/mixins.py removed
• Python 2 __all__ ASCII encoding block in particles/__init__.py removed

future package dependency removed

Replaced future.utils.with_metaclass with native Python 3 metaclass syntax:

# Before
class Dumpreader_ifc(with_metaclass(abc.ABCMeta, FieldAnalyzer)):

# After
class Dumpreader_ifc(FieldAnalyzer, metaclass=abc.ABCMeta):

Misc

• run-tests.py: removed -2 / --pycmd python2 CLI argument
• pyproject.toml: removed future from dependencies, dropped stale Python 2 comment