pwclient: Force xmlrpc client to return unicode strings

pwclient: Force xmlrpc client to return unicode strings

On python 2, the reference implementation of the XML-RPC unmarshaller
decodes strings to unicode with the selected encoding (utf-8 by default)
but it tries to re-encode the unicode strings to ascii bytes before
returning the values. If it fails, it leaves the value as unicode.

See these links for more details:

  https://hg.python.org/cpython/file/2.7/Lib/xmlrpclib.py#l878
  https://hg.python.org/cpython/file/2.7/Lib/xmlrpclib.py#l180

  https://hg.python.org/cpython/file/3.6/Lib/xmlrpc/client.py#l753

Monkey-patch the internal xmlrpclib._stringify() function only on python
2 to force it to preserve unicode strings. This allows to have similar
behaviour in both python 2 and python 3.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>
Signed-off-by: Stephen Finucane <stephen@that.guru>