iotests: Disable unsubscriptable-object in pylint

When run with Python 3.9, pylint incorrectly warns about things like
Optional[foo] because it doesn't recognise Optional as unsubscriptable.
This is a known pylint bug:

    pylint-dev/pylint#3882

Just disable this check to get rid of the warnings.

Disabling this shouldn't make us miss any real bug because mypy also
has a similar check ("... is not indexable").

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/pylintrc

@@ -17,6 +17,8 @@ disable=invalid-name,
         too-many-lines,
         too-many-locals,
         too-many-public-methods,
+        # pylint warns about Optional[] etc. as unsubscriptable in 3.9
+        unsubscriptable-object,
         # These are temporary, and should be removed:
         missing-docstring,