tox: Bump xblock12 env to XBlock 1.2.3
tox: Bump xblock12 env to XBlock 1.2.3

XBlock 1.2.3 was released on 2019-07-24, per
https://pypi.org/project/XBlock/#history

arbrandes/hastexo-xblock,arbrandes/hastexo-xblock,hastexo/hastexo-xblock,arbrandes/hastexo-xblock,hastexo/hastexo-xblock,hastexo/hastexo-xblock,arbrandes/hastexo-xblock,arbrandes/hastexo-xblock,hastexo/hastexo-xblock