CI: Don't install tomli for stubtest_stdlib (#7901)

The script doesn't import tomli

.github/workflows/stubtest_stdlib.yml

@@ -38,6 +38,6 @@ jobs:
       - 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)
+        run: pip install $(grep mypy== requirements-tests.txt)
       - name: Run stubtest
         run: python tests/stubtest_stdlib.py