gh-104050: Run mypy on clinic.py in CI (#104421)

* Add basic mypy workflow to CI
* Make the type check pass

---------

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
Co-authored-by: Nikita Sobolev <mail@sobolevn.me>
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

.github/dependabot.yml

@@ -12,3 +12,10 @@ updates:
         update-types:
           - "version-update:semver-minor"
           - "version-update:semver-patch"
+  - package-ecosystem: "pip"
+    directory: "/Tools/clinic/"
+    schedule:
+      interval: "monthly"
+    labels:
+      - "skip issue"
+      - "skip news"

.github/workflows/mypy.yml

@@ -0,0 +1,39 @@
+# Workflow to run mypy on select parts of the CPython repo
+name: mypy
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    paths:
+      - "Tools/clinic/**"
+      - ".github/workflows/mypy.yml"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+env:
+  PIP_DISABLE_PIP_VERSION_CHECK: 1
+  FORCE_COLOR: 1
+  TERM: xterm-256color  # needed for FORCE_COLOR to work on mypy on Ubuntu, see https://github.com/python/mypy/issues/13817
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  mypy:
+    name: Run mypy on Tools/clinic/
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-python@v4
+        with:
+          python-version: "3.x"
+          cache: pip
+          cache-dependency-path: Tools/clinic/requirements-dev.txt
+      - run: pip install -r Tools/clinic/requirements-dev.txt
+      - run: mypy --config-file Tools/clinic/mypy.ini