Support non-sliceable containers in uniqueness assertions

[jeet-dekivadia]

Purpose

Allow assertUnique and assertNotUnique to handle iterable containers that do not support slice indexing.

Fixes #74.

Approach

Replace slice-based duplicate detection with a shared iterator-based helper. The helper keeps a list of previously seen elements, preserving support for unhashable values while also supporting containers such as collections.deque.

Tests for New Behavior

• add regression coverage for unique and duplicate deque values with both assertions
• git diff --check
• python3 -m py_compile marbles/mixins/marbles/mixins/mixins.py marbles/mixins/tests/test_mixins.py
• python3 -m unittest tests.test_mixins.TestUniqueMixins

Checklist

• Signed and returned a copy of the CLA
• Added tests for any new features or behaviors
• Ran nox -s flake8 to make sure code style is consistent
• Built and reviewed the docs
• Added a note to the changelog

Learning

The previous implementation intentionally avoided set(container) so unhashable elements would work, but slice indexing narrowed support to sequence-like containers. A seen list keeps the original equality semantics without that indexing requirement.

Remote Linux validation

• git diff --check
• python -m compileall -q marbles/mixins
• python -m unittest tests.test_mixins.TestUniqueMixins -v