support/testing: sample_python_dbus_next: ignore F821 flake8 error

The dbus-next package uses the Python type annotation for dbus types. This is
not compatible with the python typing assumption that flake8 makes.

Exclude F821 from this line.

Fixes: https://gitlab.com/buildroot.org/buildroot/-/jobs/1687009829
partially:
support/testing/tests/package/sample_python_dbus_next.py:17:36: F821 undefined name 's'
support/testing/tests/package/sample_python_dbus_next.py:17:48: F821 undefined name 's'
support/testing/tests/package/sample_python_dbus_next.py:17:56: F821 undefined name 's'

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

support/testing/tests/package/sample_python_dbus_next.py

@@ -14,7 +14,7 @@ class SampleInterface(ServiceInterface):
         pass
 
     @method()
-    def ConcatStrings(self, what1: 's', what2: 's') -> 's':
+    def ConcatStrings(self, what1: 's', what2: 's') -> 's':  # noqa: F821
         return what1 + what2