Drop EOL Python versions, support 3.10–3.14, fix all CI failures

[Copilot]

CI was failing across all jobs because poetry.lock was not regenerated after the Python version bump to ^3.10, and several dev dependencies were either unavailable or incompatible with Python 3.13/3.14.

pyproject.toml

• Migrate from deprecated [tool.poetry.dev-dependencies] to [tool.poetry.group.dev.dependencies]
• Remove codecov (deprecated, no longer on PyPI; CI already uses codecov/codecov-action)
• Bump dev deps to modern versions compatible with Python 3.10–3.14:
  • black >=24.0, pytest >=8.0, pytest-cov >=5.0, pillow >=10.0, pytest-benchmark >=4.0, mypy >=1.0, pytest-mypy >=0.10.3

poetry.lock

• Regenerated to match updated pyproject.toml

pixelmatch/contrib/PIL.py

• Fix mypy errors surfaced by newer mypy: avoid reassigning img1/img2 from Image to MutableImageSequence
• Add type: ignore[attr-defined] for ImagingCore iteration (runtime-iterable, but Pillow stubs don't expose __iter__)

# Before: variable type changes from Image → MutableImageSequence
img1 = from_PIL_to_raw_data(img1)  # mypy: Incompatible types in assignment

# After: separate variable names
raw_img1 = from_PIL_to_raw_data(img1)

.github/workflows/test.yml

• Test matrix: [3.10, 3.11, 3.12, 3.13, 3.14]
• Bump actions: checkout v4, setup-python v5, codecov v4, cache v4
• Default Python in utility jobs: 3.13

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