Split "unicode" attributes out to a non-default fixer

[ncoghlan]

The current unicode_type fixer changes "event.unicode" references in pygame event handlers to "event.six.text_type".

It seems likely that "unicode" appearing as an attribute will be a false positive, so it would make sense to limit the default fixer to bare names, and add a separate opt-in fixer for "unicode" appearing as an attribute