Find unused stubtest whitelist entries (#4157)

* Find unused stubtest whitelist entries

This is currently a GitHub workflow that runs daily and lists all unused
whitelist entries found by running stubtest against current Python
versions on Linux and Windows. The workflow run will succeed if there
are no such entries, and fail otherwise.

In a second step, this should collate the output of the various runs and
create a PR to remove the entries. In that case, the workflow should
probably only run weekly or even monthly to keep the noise down.

Cf. #3728

.github/workflows/stubtest-unused-whitelist.yml

@@ -0,0 +1,26 @@
+name: Find unused stubtest whitelist entries
+
+on:
+  schedule:
+    - cron: '0 4 * * *'
+
+jobs:
+  find:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: ["ubuntu-latest", "windows-latest"]
+        python-version: [3.5, 3.6, 3.7, 3.8]
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Python ${{ matrix.python-version }} on ${{ matrix.os }}
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install dependencies
+        run: |
+          python -m pip install -U pip
+          pip install -U git+git://github.com/python/mypy@b3d43984
+      - name: Run stubtest
+        run: ./tests/stubtest_unused.py