ENH: Update to minimum Python support to py310

[hjmjohnson]

Summary

• Python 3.9 reached end-of-life in October 2025 and is no longer receiving security updates.
• Update black and pyupgrade to target Python 3.10+, the oldest actively supported release.

Tool	Before	After
black	--target-version py39	--target-version py310
pyupgrade	--py39-plus	--py310-plus

This aligns ITK's pre-commit config with BRAINSTools (which already uses py310 targets) and drops enforcement of syntax compatibility with an unsupported Python version.

Test plan

• pre-commit run --all-files passes on this branch with no regressions.
• Confirm no ITK Python files require py39-only syntax that would break under py310 enforcement.

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

This PR updates ITK's pre-commit configuration and Python source files to target Python 3.10+ instead of the EOL Python 3.9. The changes span linter configuration (.pre-commit-config.yaml), CI pipeline (AzurePipelinesLinux.yml), documentation (python_packaging.md), and Python type annotation modernization across several wrapping generator files. The itkConfig.py.in template is also renamed to itkConfig.template.in.py with its CMakeLists.txt reference updated.

Key changes:

• black target updated from py39 to py310 and pyupgrade from --py39-plus to --py310-plus in .pre-commit-config.yaml
• Python 3.10 type hint syntax (X | Y, X | None, built-in dict/list/type) replaces legacy typing.Union, typing.Optional, typing.Dict, typing.List, typing.Type across wrapping generator Python files
• collections.abc.Callable replaces typing.Callable in template_class.py
• Azure Pipelines pre-commit job updated to use Python 3.10
• Documentation examples refreshed with updated Python/ITK version strings
• Note: Wrapping/Generators/Python/PyBase/pyBase.i still contains a runtime version check if _version_info < (3, 9, 0) with the message "Python 3.9 or later required", even though CMake/ITKSetPython3Vars.cmake already sets PYTHON_VERSION_MIN = 3.10 and explicitly comments that these values should be kept in sync.

Confidence Score: 4/5

Mostly safe — one P1 inconsistency in pyBase.i where the runtime version guard still checks for Python 3.9 while the CMake minimum is already 3.10 and this PR adds 3.10-only syntax to the codebase.

All pre-commit, CI, and type-annotation changes look correct and consistent. The single P1 finding is in pyBase.i (not changed by this PR) where the runtime check if _version_info < (3, 9, 0) and its error message are inconsistent with CMake's PYTHON_VERSION_MIN=3.10 and with the new 3.10-only union syntax introduced in this PR — a Python 3.9 user would pass the guard but immediately hit a TypeError at import time.

Wrapping/Generators/Python/PyBase/pyBase.i — runtime version check and error message need to be updated from 3.9 to 3.10.

Important Files Changed

Filename	Overview
.pre-commit-config.yaml	Updated black target from py39 to py310 and pyupgrade from --py39-plus to --py310-plus; both active and commented-out args updated consistently
Documentation/docs/contributing/python_packaging.md	Updated Python >= 3.9 to >= 3.10 and refreshed example wheel build script invocations and output filenames
Testing/ContinuousIntegration/AzurePipelinesLinux.yml	Updated Azure Pipelines pre-commit job Python version spec from 3.9 to 3.10
Wrapping/Generators/Python/CMakeLists.txt	Updated configure_file reference from itkConfig.py.in to itkConfig.template.in.py following the file rename
Wrapping/Generators/Python/itkConfig.template.in.py	Renamed from itkConfig.py.in; removed typing imports and replaced with built-in generics and X
Wrapping/Generators/Python/itk/pyi_generator.py	Removed top-level Union import; updated two function return annotations to use str
Wrapping/Generators/Python/itk/support/types.py	Removed Optional import; updated GetCType and GetCTypeForDType return types to X
Wrapping/Generators/Python/itk/support/template_class.py	Moved Callable import from typing to collections.abc; replaced Union[str, Callable] with str

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Python runtime imports itk] --> B{pyBase.i version check\nif _version_info < 3,9,0}
    B -->|Python < 3.9| C[RuntimeError: Python 3.9 or later required\n⚠️ Message still says 3.9]
    B -->|Python >= 3.9| D[Continue module loading]
    D --> E{Python version?}
    E -->|Python 3.9| F[TypeError: unsupported operand type\nfor | in union annotations\ne.g. str | None]
    E -->|Python >= 3.10| G[Imports succeed ✅]
    H[CMakeLists.txt / PYTHON_VERSION_MIN=3.10] -.->|Build-time enforcement| A
    style C fill:#ffcccc
    style F fill:#ffcccc
    style G fill:#ccffcc

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[dzenanz]

Looks good, but preferably someone else should also review.

[blowekamp]

Updating pre-commit and updating python in wrapping seem like two rather different things.

[hjmjohnson]

Updating pre-commit and updating Python in wrapping seem like two rather different things.

@blowekamp updated the PR comment to reflect the full scope of these changes. Let me know if you want something else done.

[thewtex]

Thanks, Hans!