Move stubtest CI to GitHub Actions (#4223)

This ensures that the Python version used matches the one used in the
scheduled extraneous stubtest whitelist check.

See also PR #4206.

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

@@ -22,6 +22,6 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install -U pip
-          pip install -U git+git://github.com/python/mypy@b3d43984
+          pip install -U mypy==0.780
       - name: Run stubtest
         run: ./tests/stubtest_unused.py

.github/workflows/stubtest.yml

@@ -0,0 +1,27 @@
+name: Check stubs with stubtest
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  stubtest:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: ["ubuntu-latest", "windows-latest"]
+        python-version: [3.5, 3.6, 3.7, 3.8]
+      fail-fast: false
+
+    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 mypy==0.780
+      - name: Run stubtest
+        run: ./tests/stubtest_test.py --ignore-unused-whitelist