build: migrate packaging to pyproject.toml, require Python >=3.10

[Yuzki]

Summary

Fixes uv's "No requires-python value found" warning by migrating package
metadata from setup.py into a [project] table in pyproject.toml, and pins
the supported Python range.

Setting requires-python surfaced a real inconsistency: the dev dependency
pytest>=9.0.3 needs Python 3.10+, but CI and the old setup.py (>=3.6)
included 3.9. Since 3.9 is EOL, this standardizes on >=3.10.

Changes

• pyproject.toml: add [project] (name, version, description, readme,
  authors, classifiers, URL), requires-python = ">=3.10", and
  [tool.setuptools] packaging config (packages, include-package-data,
  package-data for the CSV).
• Remove setup.py — now redundant; all metadata lives in pyproject.toml.
• .github/workflows/python-package.yml: CI matrix 3.9/3.10/3.11 →
  3.10/3.11/3.12.
• uv.lock: re-resolved.

Notable behavior changes

• The built wheel now ships only the SanskritTransliteration package (with
  its transliteration_table.csv). The previous find_packages() also packaged
  tests/.
• Placeholder author/url ("Your Name", yourusername/…) replaced with the
  real owner name and repository URL.
• Declared support narrows from >=3.6 to >=3.10 (3.6–3.9 are EOL / untested).

Supersedes #5

PR #5 reformatted setup.py; this PR removes that file entirely, so #5 is
superseded and should be closed.

Verification

• uv build --wheel → wheel contains
  SanskritTransliteration/transliteration_table.csv, excludes tests/;
  Requires-Python: >=3.10.
• uv sync → resolves cleanly, no more requires-python warning.
• uv run ruff format --check / uv run ruff check → clean.
• uv run ty check (whole repo) → All checks passed (the prior
  unresolved-import: setuptools from setup.py is gone).
• uv run pytest → 20 passed.

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