Revert "MyPy in tests: Ignore StubTreq being a stub and not having all the attrs"

This reverts commit 7efc31d55246fdf57968862c6f36fa34c2a14585.

Greetings @meejah!  This is mostly to show you (in CI) the type error that MyPy
reports for StubTreq in the tests and that it has nothing to do with the
earlier changing changing type annotations in the code.

src/allmydata/test/test_storage_http.py

@@ -716,7 +716,7 @@ class GenericHTTPAPITests(SyncTestCase):
         """
         client = StubTreq(self.http.http_server.get_resource())
         response = self.http.result_of_with_flush(
-            client.request(  # type: ignore[attr-defined]
+            client.request(
                 "GET",
                 "http://127.0.0.1/storage/v1/version",
             ),