Make bubble sort support comparable items

[Rokesh2008]

Fixes #15234 for bubble_sort.py.

This contribution:

• models both bubble-sort functions with a Comparable protocol and TypeVar
• uses lt for comparison-sort semantics
• adds doctests and pytest coverage for mixed, non-comparable input

Tests:

• python -m pytest tests/test_sorts.py -q
• python -m doctest sorts/bubble_sort.py

Both commands pass.

[algorithms-keeper]

Closing this pull request as invalid

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