CI: Don't run stubtest_stdlib on PRs that don't touch the stdlib (#7895)

* Don't run stubtest_stdlib on PRs that don't touch the stdlib

.github/workflows/stubtest_stdlib.yml

@@ -0,0 +1,43 @@
+name: Run stubtest on the stdlib
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+      - master
+    pull_request:
+      paths:
+        - 'stdlib/**'
+        - '.github/workflows/**'
+        - 'tests/**'
+
+permissions:
+  contents: read
+
+env:
+  PIP_DISABLE_PIP_VERSION_CHECK: 1
+
+jobs:
+  stubtest-stdlib:
+    name: Check stdlib with stubtest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        # macos-11 does not have tcl/tk installed, needed for stubtesting tkinter
+        os: ["ubuntu-latest", "windows-latest", "macos-10.15"]
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11-dev"]
+      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: Update pip
+        run: python -m pip install -U pip
+      - name: Install dependencies
+        run: pip install $(grep tomli== requirements-tests.txt) $(grep mypy== requirements-tests.txt)
+      - name: Run stubtest
+        run: python tests/stubtest_stdlib.py

.github/workflows/tests.yml

@@ -92,29 +92,6 @@ jobs:
           python-version: ${{ matrix.python-version }}
           no-comments: ${{ matrix.python-version != '3.9' || matrix.python-platform != 'Linux' }} # Having each job create the same comment is too noisy.
 
-  stubtest-stdlib:
-    name: Check stdlib with stubtest
-    runs-on: ${{ matrix.os }}
-    strategy:
-      matrix:
-        # macos-11 does not have tcl/tk installed, needed for stubtesting tkinter
-        os: ["ubuntu-latest", "windows-latest", "macos-10.15"]
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11-dev"]
-      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: Update pip
-        run: python -m pip install -U pip
-      - name: Install dependencies
-        run: pip install $(grep tomli== requirements-tests.txt) $(grep mypy== requirements-tests.txt)
-      - name: Run stubtest
-        run: python tests/stubtest_stdlib.py
-
   stubtest-third-party:
     name: Check third party stubs with stubtest
     runs-on: ubuntu-20.04