Fix CI: unpin mypy target version, bump actions

mypy was pinned to python_version 3.11 while the matrix runs it on 3.12 and
3.13. Targeting 3.11 makes mypy reject numpy's stubs, which use `type`
statements introduced in 3.12, so both newer jobs failed before checking any of
our own code. Dropping the pin lets each matrix job type-check against its own
interpreter, which is better coverage than the pin gave.

Also bumps checkout/setup-python off the deprecated Node 20 runtime.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: ehsun-sh

.github/workflows/ci.yml

@@ -19,9 +19,9 @@ jobs:
         python-version: ["3.11", "3.12", "3.13"]
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
-      - uses: actions/setup-python@v5
+      - uses: actions/setup-python@v6
         with:
           python-version: ${{ matrix.python-version }}
           cache: pip

pyproject.toml

@@ -59,7 +59,10 @@ select = ["E", "F", "W", "I", "N", "UP", "B", "SIM", "RUF"]
 "tests/**" = ["N802", "N803", "N806"]
 
 [tool.mypy]
-python_version = "3.11"
+# No python_version pin: mypy targets whichever interpreter runs it, so the CI
+# matrix type-checks against each supported version. Pinning it to the minimum
+# while running on a newer interpreter makes mypy reject dependency stubs that
+# use syntax from the newer version.
 files = ["src", "tests"]
 disallow_untyped_defs = true
 disallow_incomplete_defs = true