Remove Python 2 compatibility code

[Copilot]

Project requires Python ≥3.8 but retained Python 2 compatibility layer. Removed obsolete compatibility code.

Changes

Removed six dependency:

• Replaced six.string_types → str
• Replaced six.text_type → str
• Eliminated six.PY2 conditionals (kept Python 3 paths)
• Replaced six.moves imports with stdlib equivalents

Simplified carbonlink unpickler:

• Removed dual-implementation SafeUnpickler class
• Direct module name mapping (__builtin__ → builtins, copy_reg → copyreg)

Cleaned vendored whisper.py:

• Removed izip/ifilter/xrange compatibility shims
• Direct use of native zip/filter/range

Removed future imports:

• Deleted from __future__ import absolute_import

Example

Before:

import six
from six.moves import cPickle as pickle

if isinstance(windowSize, six.string_types):
    delta = parseTimeOffset(windowSize)

After:

import pickle

if isinstance(windowSize, str):
    delta = parseTimeOffset(windowSize)

Net change: 10 files, -67/+38 lines

Original prompt

Remove anything that is only required for python2 compatibility

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