chore: bump minimum Python from 3.8 to 3.9 (#336)

* chore: bump minimum Python from 3.8 to 3.9 and adopt PEP 585 builtins

- Update requires-python, classifiers, mypy target, and black target in pyproject.toml
- Replace Python 3.8 with 3.9 in CI matrix and nightly workflow
- Replace typing.List/Dict/Tuple/Set with builtin list/dict/tuple/set (PEP 585)
- Update all 3.8 version references in README, docs, website, and install.sh
- Fix test_wayland_no_tools to mock IBus availability
- Remove List from common_types re-exports and update test

* style: reformat test files for Black 26.x and pin version in CI

Black 26.x introduced parenthesized context manager formatting.
Reformat 7 test files and pin Black>=26,<27 in CI to prevent
future drift from unpinned version upgrades.

.github/workflows/nightly.yml

@@ -175,7 +175,7 @@ jobs:
           | **Version** | ${{ steps.version.outputs.version }} |
           | **Base Version** | ${{ steps.version.outputs.base_version }} |
           | **Build Date** | ${{ steps.version.outputs.date }} |
-          | **Python** | 3.8+ |
+          | **Python** | 3.9+ |
           | **OS** | Linux (X11 & Wayland) |
 
           ---