CI: Fix mypy errors

> pdoc/test/__init__.py:1727: error: Item "None" of "TextIOWrapper[_WrappedBuffer] | None" has no attribute "write"  [union-attr]

Author: kernc

pdoc/test/__init__.py

@@ -1723,6 +1723,9 @@ def _http(self, modules: list):
                 t.start()
                 sleep(.1)
 
+                assert sys.__stderr__ and sys.__stdout__, \
+                    'https://docs.python.org/3/library/sys.html#sys.__stdout__'
+
                 if not t.is_alive():
                     sys.__stderr__.write(stderr.getvalue())
                     raise AssertionError