winreg missing from six._moved_attributes on non-Windows platforms

[benjamb]

When converting the imports of a cross-platform code base on a Linux machine, six.moves.winreg will not automatically be converted via the use of this tooling (see asottile/pyupgrade#886).

Perhaps an issue against six would be more appropriate, relevant code is here: https://github.com/benjaminp/six/blob/3b7efbcca41857da03fb01f004ccc425ab82dfbf/six.py#L323