Change the binary operators |, &, ^, - to return NotImplemented rather

than raising TypeError when the other argument is not a BaseSet.  This
made it necessary to separate the implementation of e.g. __or__ from
the union method; the latter should not return NotImplemented but
raise TypeError.  This is accomplished by making union(self, other)
return self|other, etc.; Python's binary operator machinery will raise
TypeError.

The idea behind this change is to allow other set implementations with
an incompatible internal structure; these can provide union (etc.) with
standard sets by implementing __ror__ etc.

I wish I could do this for comparisons too, but the default comparison
implementation allows comparing anything to anything else (returning
false); we don't want that (at least the test suite makes sure
e.g. Set()==42 raises TypeError).  That's probably fine; otherwise
other set implementations would be constrained to implementing a hash
that's compatible with ours.

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