package/python-twisted: add missing python3-xml dependency

Currently the test TestPythonTwisted is not broken [1] since it doesn't
test all modules provided by python-twisted package.

But TestPythonTwisted should have failed due to missing xml in
twisted/web module.

Due to this, TestPythonPy3MagicWormhole runtime test is currently
broken.

Add missing python3-xml dependency and update
sample_python_twisted.py to load twisted/web that uses xml
module.

Fixes (partially due to missing python setuptool in
python-magic-wormhole-mailbox-server):
https://gitlab.com/buildroot.org/buildroot/-/jobs/8199992646

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/python-twisted/Config.in

@@ -7,6 +7,7 @@ config BR2_PACKAGE_PYTHON_TWISTED
 	select BR2_PACKAGE_PYTHON_HYPERLINK # runtime
 	select BR2_PACKAGE_PYTHON_ZOPE_INTERFACE # runtime
 	select BR2_PACKAGE_PYTHON_TYPING_EXTENSIONS # runtime
+	select BR2_PACKAGE_PYTHON3_PYEXPAT # runtime
 	select BR2_PACKAGE_PYTHON3_ZLIB # runtime
 	help
 	  Twisted is an event-driven networking engine written in

support/testing/tests/package/sample_python_twisted.py

@@ -1,5 +1,6 @@
 from twisted.internet import protocol, reactor, endpoints
 from twisted.web.client import readBody  # noqa: F401
+from twisted.web import xmlrpc, server  # noqa: F401
 
 
 class F(protocol.Factory):