chore: pyright CI gate (public API) + Python 3.13 matrix + pyproject cleanup (Modernization Phase 3)

[MHoroszowski]

Phase 3 of issue #29 (Modernization & Ergonomics): dev-tooling

Phases 1 (#39) and 2 (#43) shipped the API ergonomics and bug fixes from issue #29. This PR closes the dev-tooling sub-bullets that were tractable in a single bundle.

What it adds

pyright strict-mode CI gate (new typecheck job in .github/workflows/ci.yml). Strict-mode pyright (already configured in pyproject.toml's [tool.pyright], was just never run in CI) now runs against the public-API surface:

• src/pptx/__init__.py — the literal entrypoint resolved by from pptx import Presentation
• src/pptx/api.py — Presentation() factory
• src/pptx/presentation.py — Presentation class
• src/pptx/util.py, src/pptx/exc.py, src/pptx/types.py — public helpers

Public-API has zero strict-mode errors after the fixes below. Strict mode on the broader codebase (chart/, oxml/simpletypes/, etc.) still surfaces ~4200 findings — those are tracked as future work and deliberately excluded from this gate.

Public-API pyright fixes:

• Presentation() and Presentation.save(): replaced if hasattr(p, "__fspath__"): p = os.fspath(p) (which doesn't narrow under pyright) with explicit pkg_file: str | IO[bytes] = os.fspath(p) if isinstance(p, os.PathLike) else p. Forge audit confirmed empirically: isinstance(x, os.PathLike) is semantically identical to hasattr(x, "__fspath__") for all real-world inputs (str/bytes excluded by both; any __fspath__ implementer matched by both via the ABC's virtual-subclass machinery).
• Added line-local # pyright: ignore[reportPrivateUsage] on the deferred imports of _Sections (from pptx.sections) and _PortContext (from pptx.parts.slide) — both legitimately consumed at this seam by Presentation.sections and Presentation.append_from. The leading-underscore convention is documented intent ("internal"); pyright sees the rule and complains regardless. Suppression is the standard escape hatch.
• Removed unused duplicate_notes_slide_for import from presentation.append_from. The noqa: F401 was hiding an actually-unused symbol; the function is still used in slide.py, just not in this seam.

Test matrix update: added Python 3.13 to the CI matrix (was 3.9-3.12).

pyproject.toml cleanup:

• Bumped requires-python from >=3.8 to >=3.9. Python 3.8 reached end-of-life 2024-10 and was never in the test matrix. PyPI users pinned to 3.8 will see a clean "no compatible version" message via wheel metadata (no runtime crash).
• Dropped the Python :: 3.8 classifier; added Python :: 3.13.
• Removed the dead [tool.black] section. The fork standardized on ruff format in v1.2.0; black is no longer used anywhere in the toolchain (no black invocation in CI, in any Makefile, or in any developer doc).

Versioning note (per Forge NIT)

Bumping requires-python is technically a breaking change for any 3.8 holdouts on PyPI. Recommend bumping the minor version (e.g. 1.0.x → 1.1.0) on the next release tag, with a HISTORY.rst callout. Pip's resolver handles the floor cleanly — 3.8 users get a clear "no compatible version" message rather than an install-then-crash.

Skipped from issue #29 Phase 3 (deferred to separate PRs)

• uv migration — replacing the setuptools build backend, adding a uv.lock, and reworking CI for uv is a significant standalone change worth its own PR.
• Ruff selection strengthening (e.g. adding B flake8-bugbear or RUF Ruff-specific rules). Trial runs surface 49 and 82 findings respectively — most are real but each requires manual resolution. Defer to a follow-up PR that pairs the rule addition with the cleanup commit.
• pytest-syrupy — the issue marks this optional.
• unittest-style test conversion — already done in this fork. Verified: grep -rln "import unittest|class.*TestCase" tests/ returns empty.

Reporting contract (CLAUDE.md §7)

$ python3 -m pytest tests/ -q | tail -3
........................................................................ [ 97%]
........................................................................ [100%]
3456 passed in 4.99s

$ python3 -m ruff check src tests | tail -3
All checks passed!

$ python3 -m behave features/ --no-color 2>&1 | tail -3
1041 scenarios passed, 0 failed, 0 skipped
3134 steps passed, 0 failed, 0 skipped
Took 0min 1.653s

$ pyright src/pptx/{__init__,api,presentation,util,exc,types}.py
0 errors, 0 warnings, 0 informations

UAT

No .pptx UAT — Phase 3 is pure dev-tooling. The diff IS the UAT: CI yaml, pyproject changes, type-narrowing fixes in api.py / presentation.py. Maintainer reviewed and signed off.

Refs #29