package/python-service-identity: add missing python-six runtime dependency

python-service-identity 21.1.0 release requires python-six at runtime [1].

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/2459841098

[1] https://github.com/pyca/service-identity/commit/705f4af829adf4d1b6e44250d8039635a73199d5

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/python-service-identity/Config.in

@@ -7,6 +7,7 @@ config BR2_PACKAGE_PYTHON_SERVICE_IDENTITY
 	select BR2_PACKAGE_PYTHON_PYASN1 # runtime
 	select BR2_PACKAGE_PYTHON_PYASN1_MODULES # runtime
 	select BR2_PACKAGE_PYTHON_PYOPENSSL # runtime
+	select BR2_PACKAGE_PYTHON_SIX # runtime
 	help
 	  Service identity verification for pyOpenSSL.