Few coverage nitpicks for the cmath module (#102067)

- partial tests for cosh/sinh overflows (L535 and L771).  I doubt
  both ||-ed conditions could be tested.
- removed inaccessible case in sqrt (L832): ax=ay=0 is handled
  above (L823) because fabs() is exact.  Also added test (checked
  with mpmath and gmpy2) for second condition on that line.
- some trivial tests for isclose (cover all conditions on L1217-1218)
- add comment for uncovered L1018

Co-authored-by: Mark Dickinson <dickinsm@gmail.com>