Fix some Mac constraint checks (#12138)

* Fix Mac platform check for dependency in py-ipython package: 'when'
  constraints in Spack directives must be Specs (either a Spec
  object or a Spec in string format)
* Fix Mac version check in py-numpy: platform.mac_ver() returns a
  3-part string as its first tuple item so the check as written would
  never pass; use Spack Version object to simplify check.
* Fix Mac version check in qt package (the check was incorrectly
  comparing ints and strings) and use Spack version object to
  simplify check.