PY3: fix xmlrpclib and email imports

Two minor fixes, should work out of box.

In the next PR I'll deal with StringIO, maybe add some encoding too (if needed).
Some of these modules seems to be supported in six - check https://bitbucket.org/gutworth/six/src/ea90cb31658b09b82b4ea376467ce8593dbb9605/six.py?at=default#cl-250. Could we use six versions?

It'd be also great to submit a PR to six with missing modules (no need to wait for PR to be merged - we may keep the `if six.PY2` in Scrapy source code).