* Fix error in definition of Im() which returned self instead of 0

* Fix error in definition of Im() which returned self instead of 0
  for non-complex arguments.

* Replace type() comparisons with isinstance() checks.

* Replace apply() calls with equivalent syntactic form.

* Simplify __hash__ definition to hash the underlying tuple.

* Use math.hypot() for more robust computation of __abs__().

* Use sorted() instead of the multi-step keys/sort/iter.

* Update comment on the cmath module.