Merged requested to merge branch/no-str-unicode-union into branch/default
This fixes the inconsistency between the fact that unions of
SomeChar were allowed, while other forms of implicit conversions between
unicode are not, i.e. the fact that
union(u'ab', union(u'c', "d")) works but
union(union(u'ab', u'c'), "d") raises AnnotatorError. For consistency, str-unicode comparisons are also forbidden.
This is an updated version of !477 (closed) but significantly simpler now that pypy barely uses unicode any more.