Further improve mypy_test.py (#7484)

* Run mypy on the 3.11 stdlib in CI, as a regression test for https://github.com/python/mypy/issues/12220
* Correct the docstring at the top of the file following the changes made in #7478
* Stop the test from crashing when run locally if there's an extra file/folder in the stubs directory.

.github/workflows/tests.yml

@@ -55,7 +55,7 @@ jobs:
     strategy:
       matrix:
         platform: ["linux", "win32", "darwin"]
-        python-version: ["2.7", "3.6", "3.7", "3.8", "3.9", "3.10"]
+        python-version: ["2.7", "3.6", "3.7", "3.8", "3.9", "3.10", "3.11"]
       fail-fast: false
     steps:
       - uses: actions/checkout@v2