chore(ci): add ruff and mypy, wire them into CI

Back the shipped py.typed promise with an enforced type check and a linter:

- Add ruff + mypy (+ types-requests) to the dev extras and dev requirements,
  with [tool.ruff]/[tool.mypy] config in pyproject.toml (mypy checks the
  package, not the tests).
- Add a lint job to the Forgejo workflow running ruff check + mypy.
- Fix the issues this surfaced: type FileLock.fd as TextIO | None, add a
  targeted type: ignore for the intentional socket.getaddrinfo monkeypatch,
  and drop an unused import.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -6,6 +6,24 @@ on:
   pull_request:
 
 jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install package with dev extras
+        run: pip install -e ".[dev]"
+
+      - name: Ruff (lint)
+        run: ruff check src/ tests/
+
+      - name: Mypy (type check)
+        run: mypy
+
   test:
     runs-on: ubuntu-latest
     strategy: