Drop Python 3.9 support

[bdraco]

Drops Python 3.9; minimum supported Python is now 3.10. Updates requires-python, the CI matrix, the mypy target and pyupgrade target to match.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (27d9cd2) to head (95ff934).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #62   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          336       336           
  Branches        16        16           
=========================================
  Hits           336       336           

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[Copilot]

Pull request overview

This PR drops Python 3.9 support across project metadata and tooling, setting Python 3.10 as the minimum supported version and aligning CI + static analysis configuration accordingly.

Changes:

• Bumped package requires-python to >=3.10.
• Removed Python 3.9 from the GitHub Actions CI test matrix.
• Updated developer tooling targets (pyupgrade and mypy) to Python 3.10, and added a breaking-change entry.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pyproject.toml	Updates packaging metadata to require Python >=3.10.
CHANGES/62.breaking.rst	Adds a Towncrier breaking-change note documenting the Python 3.9 drop.
.pre-commit-config.yaml	Updates pyupgrade and mypy hook targets from 3.9 to 3.10 (naming + report paths).
.mypy.ini	Sets mypy’s configured python_version to 3.10.
.github/workflows/ci-cd.yml	Removes Python 3.9 from the CI matrix while keeping supported versions.

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