Xmlrpc: accept data:str|bytes and require method:str

[pmhahn]

Two minor fixed to the typeshed:

1. data may be both str and bytes; actually CPython uses it both ways
2. method=None does not make much sense and should never happen :crossed_fingers:

[github-actions[bot]]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[pmhahn]

@JelleZijlstra Replaced by https://github.com/python/typeshed/pull/12734 as I no longer work for @univention and thus no longer have access to the old repository/MR.