fix py3 xmlrpc imports, close #1473

gluon/_compat.py

@@ -31,6 +31,7 @@ if PY2:
     from types import ClassType
     import cgi
     import cookielib
+    from xmlrpclib import ProtocolError
     BytesIO = StringIO
     reduce = reduce
     hashlib_md5 = hashlib.md5
@@ -94,6 +95,7 @@ else:
     from urllib.request import FancyURLopener, urlopen
     from urllib.parse import quote as urllib_quote, unquote as urllib_unquote, urlencode
     from http import cookiejar as cookielib
+    from xmlrpc.client import ProtocolError
     import html # warning, this is the python3 module and not the web2py html module
     hashlib_md5 = lambda s: hashlib.md5(bytes(s, 'utf8'))
     iterkeys = lambda d: iter(d.keys())