Copy Attributes: Use new constraints.copy() which works better with Armature constraints

Currently when copying Armature constraints with the Copy Attributes addon, it does not copy the targets. rB64a584b38a73d4745e introduced a new constraints.copy() function to the Python API which handles this correctly and elegantly with just one line of code.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D8422