Drop Python 3.9 support - #1597

What do these changes do

Drops Python 3.9 support, as agreed in #1592. Python 3.10 is now the earliest supported version. Also runs Bandit on Python 3.14.

What was wrong

• The ubuntu-latest (26.04) runner no longer provides Python 3.9, so the 3.9 unit test jobs and the mypy job fail on dev and on every PR.
• Bandit used python-version: "3.x", which now resolves to 3.15. There, pip cannot install bandit's PyYAML dependency.

How this fixes it

Commit 1, drop Python 3.9:

• pyproject.toml: requires-python = ">=3.10", remove the 3.9 classifier, Ruff target-version = "py310", mypy python_version = "3.10", remove py39 from tox, and remove the dev dependency pins that only applied to Python 3.9.
• unittest.yml: 3.10 replaces 3.9 as the earliest version on all three OSes. The ubuntu-only 3.10 entry is removed, so the matrix still has 9 jobs. PyICU 2.15 stays, because it is also the last Windows wheel for cp310 (2.15.2 and later start at cp311).
• mypy.yml: Python 3.10.
• With the py310 target, Ruff UP035 flags typing.Callable, so Callable now comes from collections.abc. Where get_type_hints() needs it at runtime (util/time.py, augment/wordnet.py), the import stays at module level with # noqa: TC003, as in util/strftime.py.
• README badges, the dev row of the version table, tests/README.md, and CHANGELOG.md ("The minimum Python version is now 3.10.", as for 3.9 in 5.3.0).
• Removes the 3.9 reason from a comment in pypi-publish.yml. Pitloom's Hatchling build hook may now be usable instead of embedding the SBOM after the build. This PR does not change that; it is your call for a separate change.

Commit 2 pins Bandit to Python 3.14, as the lint, corpus, and docs workflows already do.

Testing (Windows 11):

• ruff check, ruff format --check, flake8 pythainlp, and mypy pythainlp tests (1.19.1, target 3.10) pass.
• python -m unittest tests.core on Python 3.10.22: 752 tests, 71 skipped, 1 error. The error is test_is_within_directory_with_symlinks, because os.symlink needs extra privileges on Windows. It is unrelated to this change.
• typing.get_type_hints(pythainlp.util.time) returns the same 7 hints as on dev.

Fixes #1592

Your checklist for this pull request

• Passed code styles and structures
• Passed code linting checks and unit test